You jest, but even in the age of modern warfare, countries still actively declare war and formally notify the other country, even if a bit late, with a formal declaration. The notable exceptions being of course the USA and the USSR and Russia, which like to call their wars "police actions" and "special military operations".

I would contend that we live in an era of “5th Generation” undeclared wars between powers. I don’t personally draw a line between a missile attack and a shipment of fentanyl or cocaine which will kill citizens all the same.