From a constitutional point of view, every single war waged by the United States after World War Two was unconstitutional and therefore illegal. None of them had a proper declaration of war, passed by congress.
The federal government has given itself some cover by issuing various declarations and authorizations that fall short of an actual declaration of war. Additionally the War Powers Act, also unconstitutional, gave the President the authority to wage war without congressional approval if the war was under sixty days in duration.
None of these legalistic covers actually follow the constitution, but at least they have provided a method by which elected officials can assure the public that everything is being done properly.
Even when President Bush lied the country into war with Iraq, he used an Authorization for the Use of Force as his authority to send troops in. But that principle has escaped President Obama.
Obama has added a war with Libya to the long list of wars the United States is involved in. He claimed authority under the unconstitutional War Powers Act to initiate hostilities. And then the sixty day limitation passed, and the war did not end. President Obama still does not seek congressional approval.
And what was the reaction of congress? The House of Representatives passed a resolution rebuking the President, but it failed to halt funding for the Libya War. The one meaningful power the congress retained for itself - funding - is the one power that congress lacks the will to use.
The only thing left is for President Obama to appoint a horse to the Senate. "Hail Caesar" was the cry that brought down the Roman Republic. Will we hear a similar cry with regards to Obama?