Charlie Chaplin's "The Great Dictator" (1940) came some 12 years after the introduction of sound, but it was Chaplin's first all-talking picture, and the first in which we heard the Little Tramp speak. The dialogue turned out to be his last words; Chaplin never used the Tramp character again after this film.
In a way, the Tramp's heartfelt closing plea for peace and human brotherhood is spoken by Chaplin himself, stepping out of character to make a personal statement on the eve of the war with Hitler. The speech does not fit into the fabric of the rest of the film (as many critics noted at the time), but the passage of years has made it seem uncannily appropriate.

Chaplin conceived and filmed "The Great Dictator" during a period when an accommodation with Hitler was still thought possible in some quarters; indeed, he must have been filming when Neville Chamberlain went to Munich. But Chaplin himself had no such optimism, and his portrait of Adenoid Hynkel, dictator of Tomania, was among the first declarations of war on Hitler. The film also prophesied the persecution of the Jews, and the scenes of storm troopers terrorizing the Ghetto were thought at the time to go too far. What a sad joke that seems today.

The film itself is filled with sad, pathetic little jokes; this is Chaplin's most serious, most tragic, most human work. He did not find Hitler at all funny, needless to say, and so although he uses his own comic genius to inspire the movie, the comedy is never neutral. It is jugular, as he creates a Hynkel who is a vain, strutting buffoon, given to egomaniacal rages and ridiculous posturing. Charlie never for a moment allows us to laugh with Hynkel, but only at him, and Hynkel thus becomes the only totally unsympathetic character Chaplin has ever played. To balance him, Chaplin also plays the part of a Jewish barber who happens to be Hynkel's exact double (and who also happens to look exactly like the Little Tramp).

There are some good belly laughs in the movie, most of them involving a state visit by Belzoni Napoloni, dictator of the neighbouring nation of Bacteria. As played by Jack Oakie, Napoloni is a loud, cheerful, idiotic clown whose natural zest for a good time cuts right through Hynkel's phoney dignity.

It's during the Oakie scenes that we get many of the film's most famous comedy moments: the futile attempt to seat Napoloni on a very low chair, so Hynkel can tower over him; the negotiations during the banquet, when Hynkel says he will destroy his enemies just like this (and attempts to rip apart a handful of spaghetti, but can't), and of course the classic barber-chair scene, in which each dictator tries to pump himself higher than the other.

There are also immortal moments of Chaplin pantomime. He shaves a customer in time to classic music. As the Jewish barber, dressed in the stolen uniform of the dictator, he nonchalantly reviews "his" troops and then sits in a folding chair that collapses, causing complete confusion. And, as the dictator, he does the famous ballet with the world globe painted on a balloon.

Charles Chaplin had a problem with authority. Policemen, bosses, bureaucrats -- the powerful and the pompous all had it coming. The Little Tramp usually had his way with them all before the lights came up.
In 1938, with the winds of war swirling in Europe, Chaplin took on his biggest target -- the swaggering former tramp from Austria who lorded over Germany and its Nazi Party. The satire would be called "The Great Dictator."

It seemed, at the time, a fair fight. The most popular man in the world vs. Adolph Hitler, leader of a reeling nation. Just two years before, in "Modern Times," Chaplin had tackled capitalism as personified by Henry Ford, a union-busting admirer of Hitler. Chaplin's weapon of choice was comedy, and it was feared.
"A comic David had arisen to fight Goliath," film critic Stanley Kauffmann recalls thinking, joyously, at the time. Today, Charles Chaplin the man remains as closely linked to his leftist politics as Charlie Chaplin the comedian is to his derby hat, cane and moustache.

Nazi propagandists attacked Chaplin, saying he was Jewish (he wasn't, but he refused to deny the claim). Chaplin's homeland, Britain, vowed to ban the upcoming film, hoping not to anger Hitler. Chaplin was under pressure in his adopted home of the United States not to make the film, which he would have to finance himself via United Artists. Chaplin pressed ahead after a message of support came from President Franklin D. Roosevelt himself.

The film remains controversial to this day. Some historians have criticized the movie's mix of humour and real-life horror. That appears to be a minority view. "Comedy is the greatest way to attack anything like a totalitarian regime -- they can't stand it," author Ray Bradbury says.

Chaplin played two roles: the ridiculous-but-deadly dictator Adenoid Hynkel (Hitler) and a Jewish barber who was a dead-ringer for the despot. Actor Jack Oakie turns up as a buffoonish Mussolini.
Although the film was banned in many parts of Europe, it became Chaplin's biggest box-office success. (The tagline was "The world laughs again.") Wartime records showed that Hitler saw the film not once, but twice.

Chaplin himself had doubts about the film and almost withheld it from release when Hitler invaded France. Had he known the extent of the Nazis' evil, Chaplin said later, he never would have made The Great Dictator.

Wikipedia:

The film stars Chaplin as Hynkel and the barber, Paulette Goddard as Hannah, Jack Oakie as Napaloni, Reginald Gardiner as Schultz, Henry Daniell as Garbitsch and Billy Gilbert as Field Marshal Herring, an incompetent advisor to Hynkel. Chaplin stars in a double role as the Jewish barber (the Tramp in all but name) and the fascist dictator, clearly modeled on Adolf Hitler.

The names of the aides of Adenoid Hynkel are similar to those of Hitler. Garbitsch (pronounced "garbage"), the right hand man of Hynkel is very similar to that of Joseph Goebbels and Field Marshal Herring was clearly modelled after the Luftwaffe chief, Hermann Goering while beyond doubt the "Diggaditchie" of Bacteria, Benzino Napaloni, was modeled after Il Duce, Benito Mussolini.
Much of the film is taken up by Hynkel and Napaloni arguing over the fate of Osterlich. Originally, Mussolini was opposed to the German takeover since he saw Austria as a buffer-state between Germany and Italy. This conflict is almost forgotten today given Italy's support for Germany during World War II.