Terrorist hijackings were a major reason for sky marshals being put on planes. The Popular Front for the Liberation of Palestine famously hijacked some planes and then blew them up on camera. This led directly to Nixon putting sky marshals on planes.

The whole saga only ended after the Palestinians failed to overthrow the King of Jordan in the Black September uprising, and then traded the final hostages for another terrorist whose hijacking attempt was foiled by the crew and passengers (pilot nose-dived the plane to give the crew and passengers a chance to capture the terrorists).

https://en.wikipedia.org/wiki/Dawson%27s_Field_hijackings