The chairman of the Senate Foreign Relations Committee is pushing President Obama for a decision on how many U.S. troops will stay in Afghanistan beyond 2016 by next month’s NATO summit in Warsaw.

Sen. Bob Corker, R-Tenn., wrote in the letter that the president must use the summit in early July to show NATO allies that the U.S. is committed to Afghanistan, and said he should not delay this “critical decision” beyond that timeline.