LONDON — After ordering British forces into action in Syria, Prime Minister Theresa May faced criticism from lawmakers on Monday, with many of them seeming to resent being bypassed over the decision, more than the strikes themselves.

Speaking in Parliament, Mrs. May argued that the “limited, targeted and effective” attack had sent a vital signal that the use of chemical weapons would not be tolerated by the international community.

But the leader of the opposition Labour Party, Jeremy Corbyn, a lifelong critic of Western military intervention, questioned the justification of the strikes under international law, telling Mrs. May that she was accountable to Parliament and “not to the whims of the U.S. president.”

Several senior members of Mr. Corbyn’s party supported the principle of intervention over Syria, however, underscoring the divisions between many Labour lawmakers and their leader on several issues, including foreign policy.