In 1940, Franklin Delano Roosevelt was elected president for a third term in office.

Before this happened, it was customary for presidents to only serve two terms, but FDR's popularity was so strong that he continued on and eventually died in office during his fourth term in office.

That's when Congress stepped in and ratified the 22nd amendment which became a part of the U.S. Constitution in 1951. This limited the president to only two terms in office.

NPR:

The key provision of the 22nd Amendment reads as follows: “No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the Pre

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