A string of cautionary opinions from administration lawyers over the last two years sheds new light on President Barack Obama's halting and ultimately secretive steps to provide military support to rebels in Syria's deadly civil war.

Members of the so-called Lawyers Group of top legal advisers from across the administration argued that Mr. Obama risked violating international law and giving Syrian President Bashar al-Assad the legal grounds—and motivation—to retaliate against Americans, said current and former officials.

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