This has been asked here a few times -- while there's always more to be said, you may be interested in these older posts featuring answers from u/bigglesworth_:

https://www.reddit.com/r/AskHistorians/comments/2syu1b/how_did_world_leaders_travel_to_conferences/

https://www.reddit.com/r/AskHistorians/comments/50vdhb/how_did_the_big_three_travel_during_wwii/

https://www.reddit.com/r/AskHistorians/comments/73009l/how_did_allied_leaders_travel_safely_to_attend/

On a topic they don't address, it's worth pointing out that there's a famous instance of a military leader being targeted in the South Pacific. Yamamoto Isoroku was a fleet admiral and commander of Japanese forces in World War II; US naval intelligence had broken various of the Japanese codes and decrypted a message that he was flying from Rabaul to an airfield in Bougainville on April 18, 1943. Chester Nimitz, commander in chief in the Pacific, decided Yamamoto was worth going after, and authorized a mission to shoot the plane down. P-38 Lightning fighters of the US Army were the only planes that had the range to make the mission, and they shot down the bomber carrying Yamamoto and another also flying on the mission.