The United States declared war on the Empire of Japan on December 8, 1941, a day after the Japanese attack on Pearl Harbor. Setting aside who knew what when beforehand about the attack, there was no dispute that the attack, the first foreign one on U.S. soil since 1812, was meant as an act of war. Nevertheless, the U.S. declared war on Japan. Three days later, on December 11, the U.S. declared war on Nazi Germany and Fascist Italy, Japan's European allies. These declarations were followed up in 1942 by declarations of war on Bulgaria, Hungary and Romania.
World War II would be the last time the United States was officially in a state of war through Congressional declaration. Unlike the world war that preceded it, World War II was not billed the war to end all wars. But the United Nations, founded in its wake, was billed as an instrument to stop war. Just half a decade later the U.S. found itself in the Korean War, approved by the United Nations (after the Soviet Union, a veto-power member, walked out of the Security Council meeting). That war ended with a ceasefire in 1953. Just as U.S. troops remain in Germany and Japan since the end of World War II, they remain in South Korea since the "end" of the Korean War as well. The Korean War would be the last war officially approved by the U.N. until the collapse of the Soviet Union.
In 1991, the U.N. approved another largely American-waged war, the first Gulf War. That war, to beat back an Iraqi invasion by Kuwait, led to sanctions, years of weapons inspectors, and, eventually, in 2003, another war. Depending on who you ask, the 2003 Iraq War was authorized by the Security Council as well. The Bush Administration certainly thought so, although they didn't need it as Congress had "authorized the use of military force" in Iraq in 2002, in relation to the weapons inspections. Congress used a similar tool to authorize the use of military force in Afghanistan against Al-Qaeda forces after the attack of September 11. Though this means of authorizing war is not found in the Constitution, it is almost as old as the Republic itself.
The U.S. first authorized military force in such a way in the Quasi-War with France in 1798. Congress, in fact, has only declared war in five conflicts: the War of 1812, the Mexican-American War, the Spanish-American War and the two world wars. And since the end of World War II, Congress has accelerated its abdication of the power to declare war, formalizing much of it in the War Powers Act. America's most recent non-secret intervention was in Libya in 2011. That military action received no form of Congressional authorization at all, though Congress wasn't quite able to get itself to do much about it. Several other semi-secret and ongoing military interventions are simply noted in the president's bi-annual war powers report to Congress. The last one for the first time acknowledged military actions in Yemen and Somalia. The next one is due sometime in the next week or two.