When critics focus incessantly on the gap between the present deal and a perfect one, what they’re really doing is blaming Obama for the fact that the United States is not omnipotent. This isn’t surprising given that American omnipotence is the guiding assumption behind contemporary Republican foreign policy. Ask any GOP presidential candidate except Rand Paul what they propose doing about any global hotspot and their answer is the same: be tougher. America must take a harder line against Iran’s nuclear program, against ISIS, against Bashar al-Assad, against Russian intervention in Ukraine and against Chinese ambitions in the South China Sea.

If you believe American power is limited, this agenda is absurd. America needs Russian and Chinese support for an Iranian nuclear deal. U.S. officials can’t simultaneously put maximum pressure on both Assad and ISIS, the two main rivals for power in Syria today. They must decide who is the lesser evil. Accepting that American power is limited means prioritizing. It means making concessions to regimes and organizations you don’t like in order to put more pressure on the ones you fear most. That’s what Franklin Roosevelt did when allying with Stalin against Hitler. It’s what Richard Nixon did when he reached out to communist China in order to increase America’s leverage over the U.S.S.R.

And it’s what George W. Bush refused to do after 9/11, when he defined the “war on terror” not merely as a conflict against al-Qaeda but as a license to wage war, or cold war, against every anti-American regime supposedly pursuing weapons of mass destruction.