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In January 1957, the US adopted the Eisenhower Doctrine and pledged to prevent the spread of communism and its perceived agents in the Middle East. Although Nasser was an opponent of communism in the region, his promotion of pan-Arabism was viewed as a threat by pro-Western states in the region. Eisenhower tried to isolate Nasser and reduce his regional influence by attempting to transform King Saud into a counterweight. Also in January, the elected Jordanian prime minister and Nasser supporter Sulayman al-Nabulsi brought Jordan into a military pact with Egypt, Syria, and Saudi Arabia.

What country joined an alliance with Egypt, Syria and Saudi Arabia?
Jordan