Less than a week after casting the decisive vote against Trump-backed ObamaCare repeal legislation, McCain defended the top U.S. commander in Afghanistan from criticism by Trump, saying on twitter "Our commanders-in-chief, not our commanders in the field" were responsible for the failure in the country.

While McCain blamed past administrations for the problems in Afghanistan, he said in a statement that the lack "of successful policy and strategic guidance from Washington" had continued into the new administration.

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“I urge the President to resolve the differences within his administration as soon as possible and decide on a policy and strategy that can achieve our national security interests in Afghanistan and the region," McCain said.

McCain has promised to provide his own strategy on the 16-year-old war, warning that he would introduce an amendment in the annual defense policy bill if the White House fails to announce its own this month.

"If the President fails to do this by the time the Senate takes up the defense authorization bill in September, I will offer an amendment to that legislation which will provide such a strategy,” he said.