As Marvin and Bernard Kalb explained in their book "Kissinger," Dr. Sisco's strategy involved containing the Soviet Union's growing influence in the Middle East, convincing the Arab states that the Nixon administration was being evenhanded and coaxing Israel to withdraw from occupied Arab territory.
Although the approach did not work as planned, it eventually led to a fragile cease-fire along the Suez Canal, beginning on Aug. 7, 1970, with Jordan, Egypt and Israel agreeing to stop shooting.
Dr. Joseph Sisco was involved in diplomatic issues including Syria's invasion of Jordan in 1970 and the Mideast peace talks in 1974.
It also led to what the Kalbs called "an outbreak of temper and tension between Dr. Sisco and Kissinger" when Kissinger sought to intervene at the last minute.
"He was not intimidated by me," Kissinger said. "His preferred method was to shout me down. That's where we started and went on from there."
In July 1974, as Kissinger's chief deputy, he was dispatched to seek a solution to the Cyprus crisis that erupted after a Greek-inspired coup deposing the country's president, Archbishop Makarios, triggered a Turkish invasion five days later. Shuttling between Athens and Ankara, he helped tamp down war rumblings between the two countries.
"There is little doubt Greece would have responded to Turkey's invasion of Cyprus with its own invasion of Turkey were it not for Undersecretary of State Joseph Sisco's backstage pressure in Athens," columnists Rowland Evans and Robert Novak wrote on July 25, 1974. "In most undiplomatic language, Sisco told the Greek generals that the U.S. would abandon them to inevitable destruction if they attacked Turkey. Jolted by this unexpected threat, the military dictatorship backed down and thereby guaranteed its own fall on Tuesday."
In 1976, Dr. Sisco left government service and became president of American University. During his time there, he worked to elevate undergraduate admission standards and oversaw the construction of a long-delayed library and athletics arena, but it was not a particularly happy phase of his professional life.
Trustees and faculty members questioned the amount of time he spent on AU affairs, as well as his lucrative speaking fees and board memberships. He resigned in 1980, saying he was "absolutely fed up with fundraising."
In 1981, he launched what he called his "third career," becoming a partner of Sisco Associates, an international management and consulting firm that his wife, Jean Head Sisco, founded two years earlier.
Dr. Sisco specialized in political and economic risk analysis for U.S. and foreign companies. He also wrote op-ed pieces and journal articles, made TV appearances and lectured. He continued to speak out on foreign policy issues until a few weeks ago.
In June, in an appearance on "The NewsHour With Jim Lehrer," he defended the Bush administration's conduct of foreign affairs, including the invasion of Iraq, at a time when a bipartisan group of 27 retired Foreign Service officers was calling for President Bush's defeat.
He conceded that the administration had made mistakes but insisted that it was moving in the right direction in Iraq, however costly and painful. He predicted that North Korea would be the real threat in 2005.
Dr. Sisco's wife, whom he married in 1946 while they were students at the University of Chicago, died in 1990.
Survivors include two daughters, Carol Bolton Sisco of Riva and Jane Murdock Sisco of Bethesda; and two brothers, Paul Sisco of Bethesda and George Sisco of Broadview, Ill.
On Dec. 8, Kissinger was scheduled to present his old friend and colleague a foreign service award from the American Academy of Diplomacy. He will present the award posthumously.
"I loved Joe," Kissinger said. "He was the best type of Foreign Service officer and absolutely indispensable to me."