Although Chaplin's career and home were in the US, he was an immigrant and considered himself a man of all countries. He never became an American citizen.
During the McCarthy era, he was accused of being a communist. In 1952, while he was in England, the US rescinded his visa. The American Legion demanded that his films not be shown in the US, and until the mid 1960s, generally got its way.
Chaplin and Oona settled in Switzerland, where their 4 youngest children were born. He returned to the US only in 1972, age 83, when he was given an honorary Oscar.
Chaplin's real politics probably best shown by "Look Up Hannah" Chaplin's speech from the final scenes of The Great Dictator (1940) possibly one of the great humanist statements of all time. Make sure that you listen to (and/or read) the speech.
It's important to understand that Chaplin began writing The Great Dictator in 1938 and filming it in 1939. It was released in October 1940. The U.S. did not enter World War II until after Pearl Harbor, more than a year after the film's release. Pro Nazi organizations were active in the US through the 1930s. For example, in 1938, Henry Ford received the Grand Cross of the Supreme Order of the German Eagle for Hitler's government. In February 1939, the Nazi funded German-American Bund held a rally at Madison Square Garden in New York City which drew 20,000 people who joined together in chanting "Heil Hitler." The Great Dictator was (almost) the first anti-Nazi mainstream release in an America in which many people admired Hitler. Not only was Chaplin not praised for The Great Dictator, he was subpoenaed by a congressional subcommittee investigating "pro-war propaganda!"
And, BTW, you know who scooped Chaplin? The Three Stooges! Their short "You Nazti Spy!" was released in January 1940, eight months before The Great Dictator. But Moe, Curly, and Larry didn't have the cultural importance of Chaplin. Of course after Pearl Harbor, Hollywood turned to the almost full time production of anti-Nazi films. Casablanca was probably the best known and greatest of them. But that didn't come out until early 1943.
Chaplin was never a member of a political party. In 1942, he spoke in favor of aiding Russia in its fight against Hitler (which, of course, ultimately we did... but not until several years later). Chaplin was never a member of the Communist Party, and two of his sons enlisted and saw combat in WWII, but was attacked repeatedly from the political right. He might have survived these attacks had it not been for scandals related to his sex life. He may have played a tramp on screen but he was intensely appealing in real life and consorted with some of the great female stars of his era. But his highly publicized marriages to much younger women and his affair with Joan Berry also diminished public sympathy for him. His politics and his affairs and marriages were a one-two punch that led to his effective expulsion from the U.S.
Although Chaplin's career and home were in the US, he was an immigrant and considered himself a man of all countries. He never became an American citizen. 
Chaplin's real politics probably best shown by "Look Up Hannah" Chaplin's speech from the final scenes of 