Because the wars in question did not have adequate support from the People.
Because of the legal ramifications of a declaration of war they went to the nearly new UN for support and called the defense of Korea a Police Action. Again, all for the international politics involved. Vietnam they sort of bungled and stumbled into because of domestic politics. Should they not have gone into Korea or Vietnam?
If you are going to wait for universal support in this day and time before you defend this country you will always be just an internet Rambo. Even now, because of the leftist Democrats we can't even get a decent immigration bill passed. Do you think you could ever get them to declare war on anyone except the Republicans? They are still trying to take away the guns of responsible citizens and give illegals the right to vote. Are those the people you are waiting to declare war?
Do you mean without a declaration of war you would take no action? 9/11 you would have let stand?