A U.S. strike on Syria without a United Nations mandate would be a war crime, Noam Chomsky told the Huffington Post. The nation’s leading left-wing thinker made the comments after President Barack Obama announced that he would go to Congress to ask for authorization for an attack on Syria in response to an alleged chemical weapons attack.

“As international support for Obama’s decision to attack Syria has collapsed, along with the credibility of government claims, the administration has fallen back on a standard pretext for war crimes when all else fails: the credibility of the threats of the self-designated policeman of the world,” said Chomsky. “[T]hat aggression without UN authorization would be a war crime, a very serious one, is quite clear, despite tortured efforts to invoke other crimes as precedents.”

Before he decided on a Syria strike, President Barack Obama weighed in on the UN mandate issue in an interview with CNN. Obama said that “if the U.S. goes in and attacks another country without a U.N. mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it.” While the president has presented evidence that he says shows the Assad regime carried out a chemical weapons attack, there’s no chance of a UN resolution authorizing force. Russia and China are adamantly opposed to striking Syria.

Chomsky’s comments come as Congress is debating whether to approve Obama’s resolution that would authorize a Syria strike. The legality of the strike under the laws of war, though, is not something that has been a key point of debate.