Russia reacted to U.S. military strikes on its ally Syria Friday by cutting a hotline intended to prevent midair incidents, a response that demonstrates Moscow's readiness to defy Washington and could even put the two nuclear superpowers on a course toward military confrontation. President Vladimir Putin signaled he was ready to risk a clash with the U.S. and abandon hopes for mending ties with the U.S. under President Donald Trump, rather than accept the humiliation of standing by while his ally is bombed.