But U.S. President Richard Nixon didn't care for Allende. He and Kissinger dismissed him as a Marxist, and his association with Fidel Castro didn't help. When Chile nationalized its copper industry, that was the last straw. In September 1970, President Nixon informed the CIA that an Allende government in Chile would not be acceptable and authorized $10 million to stop Allende from coming to power or unseat him. Later, the CIA tried to convince key Chilean military officers to carry out a coup. Later revisionists claimed that the U.S. didn't directly instigate the coup. Nonetheless the events are so well documented as to appear in Wikipedia. [LINK]
You could write a thesis about the factors that led to the end of Allende: the strikes, the rise of the right wing, the CIA funding of opposition groups via the ITT Corporation, divisive public statements by the Chilean Supreme Court.. the situation was grim. But, there was a decisive turning point. On 11 September 1973, the Chilean military staged a coup to depose Allende. From a makeshift studio at the Ministry of Defense, Colonel Roberto Guillard broadcast a short speech outlining the intentions of the coup over Radio Agricultura.
At the time Allende was holed up at La Moneda, the Presidential Palace. He had a live phone line to 1010 Radio Magallane, a radio station aligned with the communists. Radio Agricultura was aligned with the conservatives and at the time was broadcasting only the national anthem. The Air Force had bombed the radio towers of Radio Portales and Radio Corporación and they were off air. On Television all stations were off air except Channel 13 which was broadcasting more statements from Pinochet. Radio Magallanes reported that army troops had encircled La Moneda and gunfights had broken out. Allende knew the end was near and he made his last speech, by phone, over Radio Magallanes. The complete text is here.
"Surely Radio Magallanes will be silenced, and the calm metal of my voice will no longer reach you. It does not matter. You will continue hearing it. I will always be next to you. At least my memory will be that of a man of dignity who was loyal to his country."Shortly after the speech aired live, Radio Magallanes and Radio Candelaria rebroadcast the recorded speech. At 10:20 AM, Radio Magallanes went off the air. Later that morning, the military stormed la Moneda and killed Allende. Radio Magallanes never returned, and the truth about the death of Allende didn't come out until 2011. More here.