The guy is like a hypnotist — a madman who can make a whole country nuts.
~ Indiana Jones on Hitler.
The Ark belongs in the Reich. Something of such antiquity belongs in Germany.
~ Adolf Hitler.
~ Hitler's salute
Adolf Hitler (also known as the "Führer") is the overarching antagonist of the first three films of the Indiana Jones series, being the unseen overarching antagonist of the 1981 film Raiders of the Lost Ark and the overarching antagonist of its 1989 sequel Indiana Jones and the Last Crusade.
Like in real-life, Adolf Hitler was the leader of Nazi Germany and is described by Colonel Musgrove as being "obsessed with the occult", hence his interest in acquiring the Ark of the Covenant and the Holy Grail. To acquire both artifacts, he hires people like René Belloq and Walter Donovan to find them for him and has many fierce officers like Herman Dietrich and Ernst Vogel, willing to accomplish his orders.
His interest in the occult and in archaeological artifacts prompted Nazi research and recovery expeditions into artifacts to either support his views of Aryan racial superiority or to be used as weapons against Nazi enemies.
Hitler met with archaeologist René Belloq in 1936 to hire his services in acquiring the lost Ark of the Covenant. After Indiana Jones thwarted Hitler's plans on numerous occasions, including his desire to claim the Ark, the Führer personally assigned Hans Degen to assassinate Jones but the attempt ended in failure.
In 1938, Hitler was presiding over a book-burning rally in Berlin outside the Institute of Aryan Culture. As he was being escorted from the forum, he came face to face with a man in a Nazi officer's uniform who held a book in his hands. Not knowing the man was a disguised Indiana Jones or that the book was the Grail diary his agents needed to find the Holy Grail, Hitler took the book, motioned one of his aides for a pen, signed the book, and returned it to the startled 'officer' before continuing on his way.
It's completely unknown what happened to Adolf Hitler afterwards, but like in real-life, Hitler is stated to have started the World War II, during which Indy continues to fight the Nazis. As the historical Hitler committed suicide in 1945 with the defeat of Germany likely and Berlin about to be invaded, it's safe to assume that this Hitler suffered the same fate.
Despite being mentioned several times and making a cameo appearance, Hitler is obviously The Heavy of at least the first and third films in the series as the Nazis (including René Belloq and Walter Donovan) were all answering to him since he's behind the plans to obtain the Ark and the Holy Grail.
Over the years, many fans have assumed that had Indy not retrieved the Ark, Hitler and all his inner circle of officers would have all died as a result of the Ark of the Covenant's powers once they opened it (Belloq only opened it at the Tabernacle to check it with the pretext of not risking Hitler's life), therefore making Indiana Jones unnecessary to the plot. In the wake of this, both Steven Spielberg and George Lucas revealed that Hitler would have been savvy enough to not open the Ark, cementing Hitler as a more threatening and calculating antagonist than Belloq.
LucasArts originally intended to revive Hitler for the story of Indiana Jones and the Iron Phoenix, an intended sequel to Indiana Jones and the Fate of Atlantis which ultimately never got to be made due the inability of selling the game of Germany for its Neo-Nazism depictions. Hitler would have been resurrected by the German scientist Matthias Jäger as a zombie with the Philosopher's Stone, only for Hitler to kill Jäger for bringing him back with no military to reclaim his power, leading to a final confrontation with Indiana Jones which ended with Hitler exploding due holding the Stone.