Another good question. We haven’t had that question of declaring war debated since the Korean War began on 25 June 1950. Korea was sold to Americans as a “police action” under the UN when it was a real war. There was no declaration of war as happened in WW2 — and there haven’t been any since 8 and 11 December 1941. [America declared war on Japan only on 8 December; it declared war on Germany and Italy on 11 December after those two countries had declared war on America.]
Nor was there a treaty of peace, only a cease-fire agreement.
This is one more Constitutional duty Congress has abdicated.