Oddly, at the end of it he argued for greater "realism" where legal restraints on the war powers are concerned. That's an argument that would have driven the anti-Vietnam War leftist subversives and traitors quite insane in view of the fact that the presidency was then occupied by the hated Richard M. Nixon. With Barack H. Obama in the White House, however, trust in the wisdom of the chief executive is the order of the day and if he is being "realistic," well then, surely there's no cause for alarm.
The start of the Korean War created a situation that was "fluid" and "unpredicatable" in spades and only a moron would have gone on TV to complain about no prior Congressional authorization. Things got sorted out at the time but I frankly don't recall the details. Be that as it may, the War Powers Act surely was drafted with that circumstance in mind and it evidenced Congress's determination that a limited compromise of its plenary authority was in order in view of the situation Truman in fact did face.
However, it's more than two years since Posner wrote his article and nothing then or since has shown that anything in Syria was an immediate threat to the United States. If anything, the greatest threat to the international order now is the continued meddling of the U.S. and its "coalition" allies in Middle Eastern matters. Apparently, we yet again wish to experiment with the lunatic idea of "regime change."
Posner failed to show then why constitutional restrictions on the president's war making should be ignored. A vague appeal to "realism" just doesn't cut it.
Didn't this stuff get hammered out in 1789, as in we thought up ways to limit the power of the chief executive (think "king") so the people and our representatives called the shots in accordance with the Constitution, elections, and duly enacted laws rather than our being subject to the whims of one man?
The problem is not that the law restricts the executive in his efforts to secure the nation. No. The problem is that the Congress has failed from the time of the attack on Libya until now to protect the nation by insisting on observance of the law. As a consequence, we are now actively engaged in military operations in a country where we do not belong with Obama's not having been required to make his case for war to the Congress and the people. This has been a stupid and lawless drift into another war that's equally as lunatic as our wars in Afghanistan and Iraq, now springing back to life and presaging further tragedy.
Apparently, this is OK with Prof. Posner.
 "The U.S. Has No Legal Basis to Intervene in Syria. But of course that won't stop us." By Eric Posner, Slate, 8/28/13.
 Would that Obama had a tenth of the understanding of foreign policy that Nixon did. His pursuit of the VC and NVA into their Cambodian sanctuaries made life a lot easier for the Colonel and his friends in the Mekong Delta in 1970, but that's another story. God bless Richard Nixon.