World War II was the last.
Truman referred to the Korean War as a “police action,” and he invoked UN authority when the Soviets walked out and the Security Council passed a resolution.
Correct. Though US forces made up the bulk of the fighting men in the theater, it was a UN mission, not a US one. The US was not at war with North Korea; South Korea was attacked by North Korea, and the UN stepped in to assist. Ditto in 1991 -- the US was not at war with Iraq, but was part of an international coalition that went in to help Saudi Arabia and Kuwait.
A formal declaration of war is a tricky political move. Declaring war necessarily means recognizing another government that you're declaring war on. In Korea, in Afghanistan, and for that matter in the Civil War, the US could not formally declare war, because recognizing the enemy government would undercut the entire rationale behind the war. The US could not declare war on the Kim regime in Pyongyang, or on the Taliban, or on the Confederacy, because to do so would be to legitimize them.
I don’t think it’s clear from the Constitution whether there must be a formal declaration, and the president has certain emergency powers that do not require immediate congressional approval, although congress does have the clear right to withhold funds.
It's clearly unclear. As commander in chief, the president clearly has the power to send troops and call up the militia in cases of invasion or insurrection. The very first president, George Washington, did just that to combat the Whiskey Rebellion.
To clarigy the gray areas in the Constitution, Congress in the wake of Vietnam passed the War Powers Act. No president has accepted its restrictions, but they've all stayed within its terms. No president and no Congress has pressed the point, so there's never been a test case.
Good. That was what I thought, but I was working mostly from memory of old news items and discussions over many years.
The Romans believed strongly in the importance of formally declaring war, and largely as a result it became a tradition in western political thought. But Latin is no longer taught in our schools and Rome and Greece are no longer honored as sources of our civilization, so those traditions have grown pretty dim. If anyone thinks of Rome, it’s liberals who say, America is a second, evil Roman empire, and conservatives who respond, No, we are not.