Congress hasn't issued a declaration of war that was signed by the president since June of 1942.

We've had wars since then.

edit,

Two recent examples from a long list:

The Persian Gulf War was admitted to be as such in April of 1991. https://www.congress.gov/102/statute/STATUTE-105/STATUTE-105...

The Global War on Terror was funded on that name in 2005. https://www.congress.gov/bill/109th-congress/house-bill/1268

Other examples exist for things like The War in Afghanistan, The Korean War, The Vietnam War, and similar. But I'm not trying to write an unprompted book here on this fine October night. :)

(Maybe Congress didn't declare those wars, but they sure seemed to accept participation in them nonetheless.)