Israel was not actually concentrating its forces. In fact, Israeli intelligence was convinced in mid-1967 that Syria was immersed in internal strife, while Egypt was tied down in Yemen, and therefore there was no immediate danger of war. This was an accurate evaluation: Neither country actually wanted a war. But by mid-May the K.G.B. could not put the genie back in the bottle.

Sadat returned to Cairo that night to find that the K.G.B. station there had issued a similar warning. He hurried to Nasser’s residence, where the Egyptian leadership had convened for an emergency meeting. Nasser didn’t necessarily believe the Soviets’ reports — but he behaved as though he did. Even if Moscow was pushing Egypt and Syria to the brink of war, he was confident that it would work to his benefit.

On May 15, the Egyptian president ordered his army to march into Sinai and then demanded that the United Nations pull its peacekeepers from the peninsula. Then he declared the Straits of Tiran, a choke point for Israel’s access to the Red Sea, closed to Israeli shipping. The Soviet Union backed these measures, believing that Israel wouldn’t dare to start a war in response and that even if the Israelis did want to strike, the United States would stop them.

Egypt’s moves came as a shock to the Israelis, who interpreted them as casus belli and began mobilizing forces to face the Egyptians. The Israeli military’s top brass put pressure on Prime Minister Levi Eshkol to approve a pre-emptive attack. The Israeli populace, imbued with a deep fear of a second Holocaust induced by Nasser’s violent threats, was gripped by anxiety. The military leadership, however, enjoying high-grade intelligence, was confident of victory.

The staff members of the Soviet embassy in Israel — Yuri Kotov among them — were less confident than their bosses in Moscow that Israel could be cowed. They summoned their agents to try to understand what was happening. At the same time, the Shin Bet activated the double agents to persuade the Soviet Union to relax the tensions.

Eshkol tried to convince the Soviets that Israel was not planning to attack Syria. In the middle of the night, in his pajamas, he received the Soviet ambassador, Sergei Chuvakhin, and proposed they go on a tour of the Israel-Syria border to see that nothing out of the ordinary was underway there. Chuvakhin declined.

When the Soviets realized that the situation might spin out of their control, they tried to head off a war. In a secret letter to President Lyndon Johnson, Alexei Kosygin, the Soviet premier, demanded that Johnson ensure Israel would not attack Syria or Egypt, while on a separate channel asking Egypt not to strike.