This Is How Many Times The US Has Officially Declared War
For much of its history the United States has been engaged in some form of armed conflict, whether it be small insurgent groups, government-backed militias, or the full formal militaries of other nations. It may come as a shock then that of all the times U.S. military forces were deployed in the nation's nearly two-and-a-half-century history, only in five wars did Washington deliver a formal declaration of war, amounting to 11 declarations against individual countries, according to Senate.gov.
U.S. military intervention has otherwise been commenced without a declaration, under such guises as "police action" — its involvement in the Korean War being a notable example of this, when U.S. and U.N. forces fought to repel North Korean and later Chinese advances (via Britannica). The fifth (and so far, last) time the United States declared war was in fact in 1942, when the government declared war on the Axis allies and satellite states of Hungary, Bulgaria, and Romania (via JSTOR Daily).
The U.S. has only given a declaration of war when it was practical to do so
As History states, the five wars in which the United States formally declared war on an adversary were the War of 1812, the Mexican-American War, the Spanish-American War, World War I, and World War II. Individual declarations of war were delivered to each Central and Axis Power government in the latter two wars. While they and many other wars in American history also had congressional support, the reason why a declaration was not issued for the rest has varied.
Some of America's adversaries had no government to which to issue a declaration, while in other cases, issuing a declaration of war was seen as redundant. For instance, from the Korean War onward, both presidents and congress have used resolutions and/or preexisting treaties to bypass formal declarations, both with debatable legality and in spite of the nominal War Powers Act (per Forbes). Meanwhile, in the deadliest war to involve the U.S., the American Civil War, a declaration of war was not issued, as it would have implied Confederate legitimacy. According to The Raab Collection, Lincoln's order to block Southern ports instead acted as a de facto declaration.