Why did English become Lingua Franca of the modern world?

English came to dominate the world during the last 50 years for 2 major reasons:

1) The United States has the largest number of wealthiest consumers. If you manufacture anything anywhere in the world you are going to want to do what you can to sell into the United States. You have to learn the language of your customers if you want to sell to them. By contrast most other countries either don't have sufficient numbers of consumers to be worth selling into (for most products) or the consumers they have don't have a lot of disposable income.

2) The United States culture is the least racist and least nationalist culture in the world. In just about every other culture and country in the world there is an inbred sense that as humans, they are superior. That is why in many countries they view "mixed race" individuals with distaste. They do not want to learn the language of a country that they know looks down on them. But the United States does not look down on any country. Even the Muslim countries, we generally don't view the Muslim people as automatically bad - we (often) view their governments as horrible and repressive, but not the people themselves. By contrast, if you go today to a country like, for example, Japan and ask them who is better a Chinese person or a Japanese person you will be told Japanese all the way. These other cultures look down on everyone not their culture and make no secret of it. So why would a Japanese person want to learn Mandarin? They know the Chinese people look down on them. Why would an Egyptian want to learn Hebrew, they know the Israeli's look down on them. And so on and so on. All these cultures prefer to use English to communicate with each other, than to engage with each other directly. They can all agree with each other that they are superior to the Americans and save face by not having to learn each other's language when they can just learn English.

it's very hard for US citizens to understand this these days but never forget that WWII was mainly started over racial arguments (superior master race, and all that garbage)

Something else to consider is the language its self may lend to the spread. As noted by John Story, English has stolen many words and ideas from other languages. Many language purists bemoan this as it makes it more difficult to learn, but the richer vocabulary allows for more ideas to be expressed concisely leading to an easier ability to express new ideas. Also since English has stolen from so many languages, its similarity to other languages makes it less intimidating to new students.

I believe three factors contributed to English being today's lingua franca:

1. as others have already mentioned, Great Britain was an "empire where the sun would never set" in the 18th century
2. US pop culture and technological advances after world war II (e.g. Elvis and the Internet)
3. English is a really simple language to learn (hardly any grammar, declension, cases, etc.)

It is important to note that English has became "The" lingua franca only recently - a few decades ago. It's only since then that English is the natural choice for the first foreign language for everybody around the world. Even in the 70's and 80's it wasn't the case. Consequently, it wasn't because of the (already non-existent) might of the British Empire and certainly wasn't because of the military might of the US - you don't learn a language based on the might of their army, especially not in peace time. Also, the cause can't be the size of the British Empire: although it helped to spread the English language within the Empire, but only this wouldn't make the English language any more special than the French or Spanish.

English becoming the one and only lingua franca started with the rapid spreading of computers and specially the internet. The popular US culture ( movies, music) also helped to strengthen this process as well as other factors (like economic power, etc) but it all started with computers. As they rapidly became indispensable in our daily lives, so did the English language.

I'd think a major factor has been that two consecutive global superpowers, Britain and then the USA, have had English as their main language has played a large part in this.

I'd back this up with Russian being a relatively common language to learn in the cold war period, whereas now it's considered maybe less useful, and this might be because Russia is way less influential than it once was.

In addition, this has made English an attractive language to learn for non-native speakers, either through cultural necessity (eg colonial India) or through utility (eg it's a good language to learn because of aspirations to move to Britain or the US, or simply because it's widely understood)

The most dominating language in science and the most learned foreign language before WWII was German. After Germany's defeat most of Europe fell under British and American occupation. Also a lot of scientists emigrated to the US. This determined the widespread use of English.

I think it is most properly represented as a mix of both.

Prior to WWII, British (UK) English did expand because:

• It was the language of administration of the Empire. If the natives wanted to be anywhere near the seat of power, they had to learn English. If they could afford to, many would send their children to study in England¹.

• Commercially, you would access to British goods through British firms, so you would need to have someone to translate the correspondence with them. And the UK was the model of a successful nation that everybody tried to imitate.

That said, trade before WWII was relatively limited (remember, ships were loaded and unloaded by hand), communication media few and slow and, for most countries, literacy rates even in their own language were quite low, so very few people would have been English speaking.

After WWII, the UK loses the Empire and the USA takes its role as a commercial and manufacturing powerhouse, and we see the development of:

• Mass transportation, which causes both an increase in trade and other exchanges (e.g. tourism, migration) ².
• Massive presence of US military bases around the world, exposing people who otherwise had no foreign contact to US English.
• Continuous improvement of school systems, which last for more years and -often- include foreign language studies.
• Explosion of mass media, which gives the public access to culture in English³. In some cases, even local artists begin using English because of the "cool" factor.
• Add to that the fact that in most countries there was already a small nucleus of English-speaking people who could help teach other people, which would have accelerated the change.

Note that I am not saying that all of the expansion of English post WWII was due to the USA⁴. I am pointing that the factors that favored the expansion of English appeared in a timeframe where the major English power was the USA, but the UK still had some influence at that time (through tourism, trade, culture, etc.).

¹ In fact, the same happened in France colonies, but as stated somewhere those accounted for less population.

² Ok, in some places like Europe, English-speaking tourism was principally from the UK and not from the USA.

³ Even something as "silly" as understanding pop song lyrics was a motivation to learning English that our parents had but our grandfathers did not.

⁴ And, conversely, probably there were parts of the world prior to WWII where English was introduced due to US influence.

The claims you cite are based on several wrong assumptions.

1. That the development of the US strongly depended on investment from Britain. British colonies are independent since 1776, and long before that they were self-sufficient (Otherwise they would not fight for independence:-)

2. British rule in India formally started much later, in 1850. It is true that there was an East India company before that, which controlled most of India since approximately 1750. So there is no evidence that money coming from India somehow played a substantial role in the development of the US.

Now on the language. I refer to the Wikipedia article:

http://en.wikipedia.org/wiki/Demographic_history_of_the_United_States

From a table in this paper we see that in 1790 Population of the US was 3.9 million, of which 2.1 were native English-speakers. The second largest part was Africans and the third - Irish. Certainly the English-speakers had a dominant position in the society, this explains why English became the language of the country. Population of the French colonies on the modern territory of the US was smaller by the orders of magnitude (sorry, I did not find the exact numbers but it is certain that it was negligible in comparison with English-speaking population).

So the reason why US is an English-speaking country is apparently that more British were wiling to emigrate and or had higher birth rate.

At the time when US was created it was certainly very far from being a great power. It became a great power only in the middle of 20-s century, and this is why English dominates today.

In the first part of 20-th century there was no English domination yet. For example, in sciences, German and French were more common. So worldwide domination of English is a relatively recent phenomenon.

EDIT. On population of French colonies in America. I found that in 1763 the ratio of English and French population was 20:1, http://en.wikipedia.org/wiki/French_colonization_of_the_Americas