When and how did the USA and the UK become allies?

America and Britain may have had their differences in Colonial Times. But the relationship today,especially since WW2 is a special relationship. Winston Churchill was probably the greatest leader during the war.Im an American and us Americans studied Churchill, I consider myself a amateur Historian, Churchill was a true leader. His relentless leadership against Nazi Germany was Legendary. America and Britain keep the world Humane. Two powerful allies who keep the world peaceful.

Britain and the United States were at loggerheads as late as the Civil War (1860-65), and perhaps even shortly thereafter.

They became allies around the turn of the 19th and 20th centuries. This was because of the rise of the Eurasian "Heartland," and of the so-called "Heartland Theories" put forth by people like Britain's HJ MacKinder (in 1904), and his German counterpart, Karl Haushofer.

Early industrialization had given a head start to "Atlantic" powers such as Britain, France, and even the United States, in the nineteenth century race for world power. But by about 1900, countries like Germany and Russia, in what was then the "heartland" of the Eurasian land mass were catching up in industrialization, and they had more people than two of the three aforementioned Atlantic powers by 1900.

At that time, the "heartland" of the "world island" (the Eurasian continent) consisted of 1) modern Russia, 2) modern Eastern European states such as the Baltics, Berlarus, Poland, the Baltics and the Ukraine, and 3) "central Europe," including Germany, Austria, ethnic Germans of the Czech Republic, and possibly Italy (in the Triple Alliance prior to World War I, and the Berlin-Rome Axis of World War II). Note that the last reference to central Europe is my contribution, and not part of the original Mackinder thesis. Despite being vastly different in size, the three components of the "heartland" have similar populations; 140 million Russians, about the same number of Eastern Europeans, and 150 million Germans and Italians.

To use a "modified" form of Mackinder's thesis, whichever power, Germany or Russia, controlled eastern Europe would dominate the combined "heartland." Control of the "heartland" by one or the other (or both in alliance) would lead to the domination of "world island," and control of "world island" could lead to the domination of the world--unless the other "islands" joined forces against them. North and South America were "united" by the Monroe Doctrine, and after 1900, England was eager to cater to the United States. England also made an alliance with Japan in 1902, and had Australia, the last big "island," as a Commonwealth country.

Germany came close to crippling Russia (and capturing Eastern Europe) in World War I, except for the defeat by the Western Allies. Early in World War II, Germany presented the specter of a true "Axis" (according to William L. Shirer in the Rise and Fall of the Third Reich), that would include Germany, Italy, Soviet Russia, and Japan. The object was to divide the lands on the "periphery" of "world Island," with Russia getting a warm water port in modern Iran or Pakistan (and a "corridor" thereto); Germany getting everything west of it, and Japan getting China, India and Southeast Asia to the east. Only Russia upset this plan by demurring; she wanted the Middle East in exchange for letting Germany dominate Europe.

The United States was mostly an isolationist power from the time of George Washington to the entry into WWII. WWI being the significant exception to that rule, which I will talk about below. But to understand the British US relationship you don't want to look at the conflicts or near conflicts between the governments, you want to look at the inter-coarse between the people. The trade and other close economic ties which all fall out of common language, common experience, and general familiarity with each other.

If you look at 1794-1795 and John Jay's Treaty I think it's indicative of the question you are asking. In the early 1790's the US and Britain were at the brink of war. Britain did not respect US neutrality in their war with France, Britain would stop American ships on the high seas and seize cargo and sailors. The US government was livid about it, and yet they negotiated and ratified a very one sided treaty favoring the British over the French. One which still allowed Britain to impress American Sailors, and stop and seize US cargo at sea (French Cargo, being carried on US Ships).

Why did the Americans do this.

• Britain was a trading culture, and so was the United States.
• Even though the two countries were nearly at war, America's relationship with Britain was still responsible for most of the US trade.
• The US did not get the income tax until (16th amendment) 1913, and was dependent on all that British trade to finance the government. Tariffs on trade was a primary source of federal income.
• US British relationship was more important to a trading nation than was the French relationship at the time.

The US Civl War was a little different. It wasn't the US picking the sides it was rather the British. In this war the British would threaten to come in on the Confederates side twice. British industrial age clothing looms demanded cotton which the south could provide, that was the motivation. Lincoln and his secretary of state (William Henry Seward) would do a masterful job in keeping the British out.

• Trent Affair - fall of 1861 - The United States Navy boarded a British ship and arrested/detained two Confederate diplomates headed to London. James M. Mason and John Slidell. Britain would threaten to enter the war on the side of the Confederacy over the violation. Lincoln would apologize and have the two detained confederate diplomates turned over to the British, thus averting the British intervention.

• Recognizing the Confederacy, summer of 1862After the battle of second Manassas another Southern Victory, Britain threatened to recognize the South as the continuation of the war was causing considerable hardship among their industrial segments, and progress seemed to be stagnating. This would have brought on a full scale war with the US, including a Union Invasion of Canada the US threatened. Ultimately Lincoln would play politics with this threat. They made the war about Slavery. Issuing the Emancipation Proclamation and this along with the Union Victory at Antiedum would keep the British out of the war. Britain had outlawed slavery(1833) and they could not intercede on the side of slavery politically.

Lincoln Wrote a famous letter to the industrial workers of Manchester

I know and deeply deplore the sufferings which the working people of Manchester and in all Europe are called to endure in this crisis. It has been often and studiously represented that the attempt to overthrow this Government which was built on the foundation of human rights, and to substitute for it one which should rest exclusively on the basis of slavery, was likely to obtain the favor of Europe.

Through the action of disloyal citizens, the working people of Europe have been subjected to a severe trial for the purpose of forcing their sanction to that attempt. Under the circumstances I cannot but regard your decisive utterances on the question as an instance of sublime Christian heroism which has not been surpassed in any age or in any country. It is indeed an energetic and re-inspiring assurance of the inherent truth and of the ultimate and universal triumph of justice, humanity and freedom.

I hail this interchange of sentiments, therefore, as an augury that, whatever else may happen, whatever misfortune may befall your country or my own, the peace and friendship which now exists between the two nations will be, as it shall be my desire to make them, perpetual. —Abraham Lincoln, 19 January 1863 There is now a statue of Lincoln in Manchester, with an extract from his letter carved on the plinth.

So even while the Governments didn't get along at times, the people did even when tensions where high. This fostered and fueled a strong economy relationship which transcended government policy.

WWI is a demonstration of this transcending economic relationship and explains the US siding with the British, as it was a consortium of US banks(2000) organized by J.P. Morgan; which first supported the British and France financially(*). Morgan had taken over his families investment Banking business (House of Morgan) in 1913 when his father died and now was in control of the bi continental banking empire. His banks branches were in New York, London, and Paris and he made multiple loans in the hundreds of millions of dollars scale to Britain and France. Likewise the US maintained Neutrality during the early stages of that war. The US Steel and Manufacturing, and agricultural industries were all up significantly supplying goods to Britain and France. The outstanding loans alone would make the US entry into WWI on the side of Germany unimaginable, and the US entry on the side of Britain a forgone conclusion when the war languished on and the Allies began to receive unsecured loans from US banks. (see business considerations)

The US British relationship is today a governmental one. Britain is a five eye's ally of the United States which puts in on a special tier beyond other close allies. But historically the US/British relationship is based upon economics which start with the populations of both countries, and not the governments.

(*) A Private Bank at War: J.P. Morgan & Co. and France, 1914–1918 https://www.cambridge.org/core/journals/business-history-review/article/private-bank-at-war-jp-morgan-co-and-france-19141918/6C62E2400476C7565DF288B19F04756A

One thing that seems to have been overlooked here is that between the time of Trafalgar (1805) and 1914, the Royal Navy was the only outfit that could guarantee the safe passage of trade anywhere. After the War of Independence the fledgling USA needed RN protection to get goods in and out. Aside from the time of the war of 1812 the USA thus clung to the relationship.

The only viable alternative for the USA would have been alliance with France which had been rendered impotent by Napoleon's wars, and would have far more feeble economic growth in the nineteenth century than Britain and Germany. Even the maintenance of the French Far Eastern Empire was dependent on British coaling stations in the Middle East and India.

But an important thing about the nineteenth century and the thing that greatly assisted growth almost everywhere was that 1815 to 1914 was a century of peace. (Apart from localised wars such as Crimea and South Africa) Britain was the world's first industrial nation, from the early 18th century. Germany did not get going till about 1845. Hence it was very much in America's interest to have a trading partnership with Britain, much as it is in our interest today to have one with them.

From before the Revolution and then onward both countries enjoyed extensive trade relationships. That is a more or less permanent feature of the relationship that can't be overlooked and shouldn't be underestimated. Because trade relationships also mean personal relationships and inter-country travel. British subjects and American citizens have been present in each others' countries more or less since Jamestown. So, from the standpoint of relations between the peoples of the two nations, there have always been close relationships accentuated even more because of the ubiquitous blood relationships--until the 20th century most Americans claimed descent from the UK. The differences were more between the governments and while I do not attempt to minimize that I do mean to point out that, after the Revolution, an ultimate alliance and close relationship between the United States and the United Kingdom was not only inevitable, it was already largely in place. The UK is the mother country to the US. It is a family and, in the weird ways that families fight, they will also forget and ultimately cling to one another. We've always been close, it just hasnt always been clear.

US/UK relations remained strained through the US Civil War. During the US Civil war, Britain was officially neutral, but a senior British official who may have been suffering from some form of degenerative insanity crossed the line and arranged for the Confederacy to take possession of British warships, notably the Alabama- this violated Britain's neutrality, and obligated the UK to participate in the final settlement of war claims.The US negotiator was instructed to ask for Canada as reparations, but settle for Jamaica.

From the American point of view, the foremost reason for the breach was the construction and refitting of Confederate warships by British shipbuilders during the American Civil War (1861-1865). American politicians argued that such behavior violated Britain's official neutrality, and demanded that the British government make financial restitution--collectively known as the Alabama claims after the most successful of the Confederate ships. NY Times

Several rounds of negotiations took place until Britain sent George Robinson, Viscount Goderich. Robinson was clever enough to approach American Masons and get to know them socially before the negotiations. As a result, in the Treaty of Washington the US gave up almost all their claims, the British finally withdrew their troops from their fort in Puget Sound (thus fulfilling the last clause of the treaty of Paris that settled the revolutionary war). Viscount Goderich's skill at negotiating changed the fundamental underpinnings of the US/UK relations.

I'm not overly fond of the big man theory of history, but George Robinson is a serious candidate.

(aside: I don't have good sources for most of this; I did this research nearly a decade ago and I'm not sure where to look anymore. I can give you enough of the story to find sources on your own, but I admit that I should eat some crow.) (Aside #2: Thanks to @semaphore for reminding me that it actually isn't that tough to find sources if you've a mind to do so. His sources are actually better than my originals; please read his comment)

Several reasons (my answer is based mostly on the book "Dreadnought" by Robert Massie).

The English and Americans spoke the same language and, in those times, their cultures were much closer to each other than, say, English and French or English and German. The increasing wealth of the United States generated increasing respect on the other side of the Atlantic. A strong advocate of US-English friendship had been Joseph Chamberlain. It was part of his dream of building a global community of English-speaking nations (which sort of happened in the form of the Commonwealth, but without Americans).

Around the end of the XIXth century the British realised that they would not be able to defeat the American navy in the (already considered unlikely) event of a war, and they modified their military doctrine from "Royal Navy must be 10% stronger than any other two navies combined, including the US navy" to "RN must be 60% stronger than the German navy" -- effectively admitting that they will not be able to beat the USN anyway, so let's give up trying (the English relocated their bases and RN assets in West Indies accordingly). (after David K. Brown, "The Grand Fleet").

Already hinted in the 2nd paragraph, another major factor has been the growing threat from the Germans. The UK needed allies, and friendship with Americans seemed like a natural thing, due to cultural affinity and lack of direct competition (Americans had no colonial interests in Asia or Africa).

It pretty much goes back to the Monroe Doctorine (1823). The Brits suggested a joint statement between the two countries, backed by the British Navy and the logistical advantages the US armed forces enjoy in the Western Hemisphere. The Monroe administration decided to just make the statement themselves, since it would be in the Brit's interests to enforce it with their navy anyway.

Wikipedia refers to this as a "precursor to the Special Relationship", which is I guess the point I'm trying to make. 100 years of this kind of tacit military alliance got the two sides used to thinking of themselves as allies (particularly the navies). When the US Navy surpassed the British, all the Brits really needed to do was make damn sure the US kept thinking the same way.

This can be seen in action in the run up to WWII. Japan had been a UK ally in part for naval logistical reasons since 1902, they fought together in WWI, and the UK re-upped that treaty twice. However, when it became clear that Japan and the USA were starting to become antagonistic toward each other in the 1920's, it also became clear the UK had to chose one. For reasons that I think became obvious two decades later, this was no choice at all. The UK dropped the treaty.