The Cold War "hotline" between the United States and the Soviet Union was borne out of the harrowing Cuban missile crisis, as leaders recognized the need for a direct channel of communication.
The tense days of October 1962 invited dangerous speculation on each side about their adversary's intentions, as messages from Moscow and Washington took hours to be translated and delivered.
A letter from October 26, 1962, in which the Soviets laid out a possible way out of the confrontation, was received by the US ambassador to Moscow at 9:42 am Washington time, but after being translated, transmitted and encrypted, it was after 9 pm by the time the note was in the hands of the State Department.
"World peace was hanging by a thread, but it took nearly 12 hours to deliver a message from one superpower to another," wrote Michael Dobbs in his seminal history of the crisis, "One Minute to Midnight."
Even when messages arrived, "they were couched in the opaque language of superpower diplomacy, which barred the writer form admitting weakness or conceding error."
To speed up communication and reduce the risk of nuclear war, the two governments put in place a "red phone" on August 30, 1963. In reality, it was neither red nor a telephone but a cable line that could transmit written messages.
In the 1970s, a satellite phone line was added, which also allowed for documents, maps or photos to be shared between the two governments.
The White House and the Pentagon have always kept secret exactly how many times the "red phone" has been used. Leaders employed the hotline during the Arab-Israeli wars of 1967 and 1973, and again with the Soviet invasion of Afghanistan in 1979.
Other lines of direct communication were later installed between Moscow and European capitals. In 1996, China established a "red phone" with Russia, then with the United States two years later. In 2005, India and Pakistan did the same.
In September 2011, the United States proposed opening a direct line with Iran to avoid a possible conflict erupting over the country's disputed nuclear program. Tehran declined the offer.