The Carter Center had been deeply involved in Syria since the early 1980s, and we shared our insights with top officials in Washington, seeking to preserve an opportunity for a political solution to the rapidly growing conflict. Despite our persistent but confidential protests, the early American position was that the first step in resolving the dispute had to be the removal of Mr. Assad from office. Those who knew him saw this as a fruitless demand, but it has been maintained for more than four years. In effect, our prerequisite for peace efforts has been an impossibility.

Kofi Annan, the former United Nations secretary general, and Lakhdar Brahimi, a former Algerian foreign minister, tried to end the conflict as special representatives of the United Nations, but abandoned the effort as fruitless because of incompatibilities among America, Russia and other nations regarding the status of Mr. Assad during a peace process.

In May 2015, a group of global leaders known as the Elders visited Moscow, where we had detailed discussions with the American ambassador, former President Mikhail S. Gorbachev, former Prime Minister Yevgeny M. Primakov, Foreign Minister Sergey V. Lavrov and representatives of international think tanks, including the Moscow branch of the Carnegie Center.

They pointed out the longstanding partnership between Russia and the Assad regime and the great threat of the Islamic State to Russia, where an estimated 14 percent of its population are Sunni Muslims. Later, I questioned President Putin about his support for Mr. Assad, and about his two sessions that year with representatives of factions from Syria. He replied that little progress had been made, and he thought that the only real chance of ending the conflict was for the United States and Russia to be joined by Iran, Turkey and Saudi Arabia in preparing a comprehensive peace proposal. He believed that all factions in Syria, except the Islamic State, would accept almost any plan endorsed strongly by these five, with Iran and Russia supporting Mr. Assad and the other three backing the opposition. With his approval, I relayed this suggestion to Washington.

For the past three years, the Carter Center has been working with Syrians across political divides, armed opposition group leaders and diplomats from the United Nations and Europe to find a political path for ending the conflict. This effort has been based on data-driven research about the Syrian catastrophe that the center has conducted, which reveals the location of different factions and clearly shows that neither side in Syria can prevail militarily.