In mid May 1967, the Soviet Union issued warnings to Nasser of an impending Israeli attack on Syria, although Chief of Staff Mohamed Fawzi considered the warnings to be "baseless". According to Kandil, without Nasser's authorization, Amer used the Soviet warnings as a pretext to dispatch troops to Sinai on 14 May, and Nasser subsequently demanded UNEF's withdrawal. Earlier that day, Nasser received a warning from King Hussein of Israeli-American collusion to drag Egypt into war. The message had been originally received by Amer on 2 May, but was withheld from Nasser until the Sinai deployment on 14 May. Although in the preceding months, Hussein and Nasser had been accusing each other of avoiding a fight with Israel, Hussein was nonetheless wary that an Egyptian-Israeli war would risk the West Bank's occupation by Israel. Nasser still felt that the US would restrain Israel from attacking due to assurances that he received from the US and Soviet Union. In turn, he also reassured both powers that Egypt would only act defensively.

What country warned Nasser that Israel wanted to attack Syria?