The last time United States Congress voted on whether or not to formally declare war was in 1999 against the Federal Republic of Yugoslavia.It received 427 Nays to 2 Yeas; the Yeas being Gene Taylor (Mississippi, Democrat) and Joe Barton (Texas, Republican)https://www.c-span.org/program/house-proceeding/house-session/134470The last time before this was H.J.Res. 321 declaring war against the Government of Rumania in 1942, which passed 361-0.