Because Article I, Section 8 of the U.S. Constitution kind of requires it.
The U.S. was effectively at war with Germany long before Germany formally declared war in 1941. The U.S. was supplying arms and munitions to Great Britain long before that, and they were being transported to Britain under escort by U.S. naval vessels in the North Atlantic.
Assange "waged war on us"? LOL. You must be kidding me. We can't even figure out what the hell our military is doing in places like Iraq and Afghanistan, and you're going to put that on him?
Article 1 section 8 does not impose a requirement to declare war if someone declares war on us. It assigns a power to Congress. That power does not mean that we are devoid of the power to defend ourselves pending any necessary or unnecessary formal declaration of war by us.
Yes, Assange waged war on us. There is no other description for what he did. You sure you’re on the right website?