An article I read recently opened my eyes on the subject of "declared war."
It made the point that the Constitution meant that Congress could "declare" that a "state of war" already existed, and that then it was okay to go to war. Changed my perspective a bit.
I thought the answer was 1941 against Japan and by extension the Axis allied with them. But I didn't know what @Attikos said about 1942 and Romania. Very interesting.
But I wonder, are you talking about any nation? It seems both @Attikos and I were concentrating only on America.
There are some that say that Spain bombed the USS Maine in Havana Harbor, but that could easily have been a false flag operation to trick Americans into wanting war. False flags have been used throughout history.
In fact, 9/11 looks to have been a false flag operation. It certainly doesn't look like Bin Laden or Al Qaeda did it. Simply High School physics proves controlled demolition and that proves inside job.
@Attikos came up with a real nice answer.