He must decide whether to renew a presidential waiver, signed by his predecessor, Barack Obama, that expires on June 1. The six-month waiver delays implementing a law passed by Congress in 1995 that requires the US to recognise occupied Jerusalem as Israel's capital and relocate its embassy there from Tel Aviv.
It is a law every president for the past 22 years has baulked at. It would pre-empt the Oslo accords and negate Washington's assumed role as "honest broker". Carrying out Congress's wish would deny the Palestinians East Jerusalem, the only credible capital of a future Palestinian state.
But equally significantly, the law would recognise Israel's efforts to claim sovereignty over the Old City's holy places, especially the incendiary site of Al Aqsa mosque. That could provoke a conflagration both locally, among Palestinians, and more generally in the Middle East.