What does the US law say about such strikes? Ideally, presidents have preferred to ensure that they have domestic support from Congress before resorting to the use of military force.

George Bush and George W Bush secured support for their wars in Iraq, but Bill Clinton launched strikes in Libya and Kosovo without congressional approval.

Barack Obama joined a multilateral military intervention in Libya that had the backing of the United Nations Security Council, a second possible strategy for securing support for intervention, but not one that guarantees that foreign interventions will be popular at home.

Given the uncertainty surrounding military strikes, though, it would be wise to consider non-military options very seriously before resorting to the use of force.

Trump may choose to turn, once again, to the Kremlin for help in pressuring Assad to get rid of chemical weapons. So far, though, Russia has refused even to attribute responsibility for the recent attacks to the Assad regime. And whether Trump has any leverage over Russia is far from clear.

Mr Trump will also know be intensely aware that the personal costs he may suffer of inaction in Syria could be considerable.

Absent the strong support of Congress, the public, and international allies, it is a very risky calculation for a President who cares a great deal about his domestic popularity. How he responds may tell us a great deal about the next four years.