Today marks John Bolton's first day as President Donald Trump's national security advisor.

Bolton is replacing previous National Security Advisor H.R. McMaster.

Last month, President Trump announced he was replacing the three-star general.

Bolton served as United Nations ambassador under former President George W. Bush.

He's President Trump's third national security adviser.

Bolton's first day comes on the heels of the latest missile strike and deadly chemical attack in Syria.

The National Security Council is holding a meeting today to discuss the attack and possible actions.