Why no assasinations?

What's with this taboo on assasination that the US government seems to have?  Why is it such a bad idea to send a covert unit to kill the leader of your enemies instead of carpet bombing the whole damn country, causing the deaths of hundreads of troops and civillians and costing billions of dollars? 

Assasination is a fuck good idea!  Why discount it in whole?

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The issue of assassination is "opening a gate of hell". If you start assassinating enemy leaders, what will stop them from assassinating you? Maybe, you'll get a few guys, then everyone will be hiding in their "Dick Cheney Mk 6" Command Bunker, afraid to go out.