When did the practice of formally "declaring war" cease and why?

Pearl Harbor probably has some influence here. The Japanese wanted to present an official declaration of war, but screwed up with their timing. They were half an hour late; they declared war after the attack.

Which was actually worse. Now it was not only a sneak attack, but a planned sneak attack that would have given the defenders exactly as much chance: nothing.

After WW2 some wars were declared, but in general not very often. Surprise is far more valuable than good manners. Today if a war is actually declared, surprise doesn't matter anyway. For example, the Falkland were invaded without a declaration of war by Argentina, but England had all the time of the world for any diplomatic nicety you can think of.

There are some exceptions, such as the 2005 Chad declaration of a state of war with Sudan.

The reason why it seems that there are much less declarations of war in the second part of the 20th Century, is that most of the wars involving developed countries were not formally a war between two countries. Indeed, most were formally

• Civil wars (e.g. Vietnam, Lybia, Yougoslavia),
• (De)Colonisation (e.g. Vietnam, Algeria).

Were no other state was officially present, or not recognised. In civil wars, other powers were sometimes supporting one of the side, but not officially at war themselves.

Nevertheless, there are still official declaration of wars, like the Chad against the Sudan in 2005 (and until 2010).

Due to the complexity of UN regulation, the formal declaration of war is not so trivial. The wars against terrorism are not officially against states, and thus no such declaration can be done. And generally the types of wars being fought have considerably changed.

One notable exception is that the USA should have declared war against Irak in 2003. But my recollection from the time, is that it wasn't made, to profit from a surprise attack, similar to the Japanese in 1941.

I think the main reasons are:

• to attack surprisingly, even if the aggressor coordinates first shots with the minister,
• to take off the guilt from the aggressor. For example, Nazi Germany in September 1939 were counter-fighting the Gleiwitz incident, as self-defence of course, and Poland was made the aggressor,
• to cheat the democratic people that this is not the war but "military action" so they are less opposed,
• especially when we would now be attacked by the country that we perform a "military action" against, this would be then the act of war (we're not at war now),
• to bypass internal regulations. For example in the USA the war is declared by Congress, but the President can undertake "necessary military measures" eg. against Serbia or Libya,
• to protect our spies against death penalty, which is in many countries made only during a war,
• to prevent mass-media reporting our officials doing this "old-fashion stuff" and "playing diplomatic games" for tax-payers' money, while "many people are about to die",
• to prevent opposition against "the governing party lead our country to war",
• to ensure everyone else that we are also wanting peace and war is ugly,
• Russia will not intern our ships when we are in military action with Serbia. That's also good for Russia, because they do not have to think about every US ship, A/C etc., how long it is in Russia and 48 hrs. have already passed or not,
• we don't bother other Powers,
• it's easier to find allies for "military action" than for war (from all the reasons above),
• it's easier to have another country with "friend neutrality": you don't look what we do against Ruritania, and we don't look what you do with Curaguay,
• it's hard to keep war in the whole world. This "military action" should be kept local, so we do not bother our opponent's ships and A/C -- but we should if this would be war,
• during military action we can still perform trade with attacked country,
• after the war we usually want some contribution, some territories etc. So we make peace with leaders that we presented declaration of war. In military action we don't want anything (it would be bad and imperialistic if we kill people to get money and territories), but we want to change the officials to our spies or fans (who then lower customs, give mining concessions, buyl 10000 ships from our shipyards etc.). During the war the officials can become heroes. So it's easier for the defending country to change its government than to just surrender the war,
• (last not least) because it is in fact in modern world not necessary.

In fact formal declaration of war in many countries brings many practical legal consequences, which may include:

• imposition of martial law

• extraordinary powers for the head of state

• ban on political parties and political activities as well as strikes

• limitation of rights of foreign nationals, especially those of the enemy state

These consequences are not often desirable if a war does not affect the general population of the involved party. It is not needed to invest the president with special powers domestically, the enemy nationals living in the attacking state usually either neutral or supportive of the invasion (otherwise why they do not live at home?), the factories do not need criminally prosecute workers who miss workdays to operate normally etc.

The attacked party usually does not declare war because it makes possibility to avoid the full-scale conflict more difficult. Not having declaration of war makes easier for the attacking party to withdraw from the conflict.

Also declaration brings the impression that the war is conducted by a certain power rather than "international community" which is often desirable due to political reasons.

In his judgment in the Tokyo war crimes tribunal, Justice Radhabinod Pal claimed that while in the west there was a convention of declaring war before the resumption of hostilities, the east did not have anything similar. He also provided a number of examples of wars that had been fought without ever declaring war, which is available in the report. So it can be said that even before 1945 the rule was not followed very strictly.