The 22nd Amendment to the U.S. Constitution was enacted in March, 1951. It states that no one can be elected to the office of President of the United States for more than two terms, which amounts to two four-year periods of eight years total. The 22nd amendment also restricts someone who comes into the office of President, say as a vice president who takes the presidency due to death, and impeachment of the sitting president, from being elected to more than one additional term.
The premise behind the 22nd amendment is traced back to the very first US President, George Washington, who refused to accept a third term in office. It is generally seen as a term limit that prevents the establishment of a government structure similar to a monarchy in principle, if not in actual fact. After Washington refused to accept a third term in office, almost 150 years passed before a president had the opportunity to face the same dilemma.
Franklin Delano Roosevelt became President of the US in 1932, and went on to serve a third term. One of the key reasons Roosevelt was elected to a third term in 1940 was that the US had just emerged from the Great Depression, and Hitler was threatening Europe. A change of leadership during this crucial time in history for the nation was seen as unwise. By 1944, however, with the world engulfed in war, the same conditions persisted, and Roosevelt ran for and was elected to a fourth term in office.
Roosevelt lived less than 100 days into his fourth term, finally succumbing to what at the time was considered polio, but is now thought to be Guillain-Barre syndrome that had kept him in a wheelchair since the age of 39. Vice president Harry Truman then took his place as president. Hindsight of the US Congress after the war ended led to a push to make term limits the official law of the land. Truman was the one exception to the 22nd amendment, as it allowed for the sitting president at the time of its passing to run for a third term. Though Truman was eligible to run for a third term in 1952, he chose not to do so.
Some critics of the 22nd amendment claim that it adversely affects foreign policy. Dictators like Saddam Hussein who have no term limits can often wait for the US Presidency to change hands before they take actions they know would be harshly responded to by the current sitting president. Another example of this effect was when US hostages held by Iran were released the very moment after Ronald Reagan took the oath of office in 1981, apparently in an attempt to snub President Carter who had tried to rescue them, and to court President Reagan's favor going forward.