Why was Salvador Allende elected and subsequently overthrown?

The late Chilean president Salvador Allende was elected in a compromise decision of the Chilean Congress after no candidate in the 1970 election was able to secure a majority of the popular vote. Under the Chilean constitution, the two candidates receiving the greatest number of votes in the presidential election—in this case, Allende of the Socialist Party and his principal opponent Jorge Alessandri of the National Party—would be submitted to a final decision of Congress. As no political party controlled a majority in the Chamber of Deputies, the Christian Democratic Party, whose candidate Radomiro Tomic had been eliminated in the national election, was in the position of "kingmaker." After Allende signed a pledge not to nationalize private property and to follow Chilean law, the Christian Democrats threw their support to him and combined their votes with those of the Socialists to install Allende as president.

Allende was extraconstitutionally removed from office in 1973 through a putsch led by the Chilean Army and supported by the Navy, Air Force, and Carabineros following several months of increasing domestic tension over his policies and specifically charges that he had violated the pledge he had earlier made that secured his election.

Several notable touch points of this increased tension are listed here:

• By 1972—two years after Allende's election—Chilean real wages had dropped dramatically and inflation had hit 150 percent. Allende's popularity had fallen well below 50-percent in national polling.
• On June 29, 1973, the 2nd Armored Battalion of the Chilean Army revolted and laid siege to the presidential palace. This abortive coup was suppressed by loyalist elements of the Army led by Gen. Augusto Pinochet.
• In early August, the Supreme Court of Chile issued a public statement complaining that Allende had refused to enforce more than 7,000 court orders and judicial rulings since taking the presidency three years earlier.
• In late August, the Chamber of Deputies enacted a resolution that declared there had been a "legal and constitutional breakdown of order" and requesting that the armed forces "put an immediate end to all situations herein referred to that breach the Constitution and the laws of the land."

The following month, in September, Gen. Pinochet—citing the earlier resolution of the Chamber of Deputies—ordered the armed forces into action and surrounded the presidential palace, ordering Allende to resign the presidency. After Allende refused, the Army and Air Force opened fire on the building, leading to Allende's eventual death under unclear circumstances.

Who was Salvador Allende?

Salvador Allende was the President of Chile from November of 1970 until he was overthrown by a coup in September of 1973.  He died during the coup, killing himself as the military attacked the presidential palace.  The cause of his death had been in dispute, but an autopsy in mid-2011 proved that he did in fact die by his own hand.

Allende is important because he was the first democratically elected Marxist leader in the Americas.  Since this was during the Cold War, the United States government worked to try to prevent from becoming president and then tried to get him removed from office once he was in power.