Yes, Congress Can Authorize War Without Formally 'Declaring' It

There are already two general positions about such an authorizing resolution. One view regards it as lawless, on the grounds that any use of force is illegal unless Congress has passed a bill "declaring war." John Nichols of The Nation, for example, sees every military conflict since World War II as illegal, because presidents since Franklin Roosevelt "have not obtained the formal declarations of war required by the Constitution."