quora 上有人问，苏联对日宣战是人类历史上最后一次国对国宣战 https://www.quora.com/Why-does-the-United-States-no-longer-declare-war
It's not just the United States. It is said that no country has formally
declared war on another state since Russia declared war on Japan in 1945.
After that the United Nations Charter defined the legal framework to apply
in international law after war is declared on another sovereign state. All
countries want to avoid following those rules so they simply go ahead and
have military conflicts without ever declaring war.
