As political instability grew in Syria, delegations from the country were sent to Nasser demanding immediate unification with Egypt. Nasser initially turned down the request, citing the two countries' incompatible political and economic systems, lack of contiguity, the Syrian military's record of intervention in politics, and the deep factionalism among Syria's political forces. However, in January 1958, a second Syrian delegation managed to convince Nasser of an impending communist takeover and a consequent slide to civil strife. Nasser subsequently opted for union, albeit on the condition that it would be a total political merger with him as its president, to which the delegates and Syrian president Shukri al-Quwatli agreed. On 1 February, the United Arab Republic (UAR) was proclaimed and, according to Dawisha, the Arab world reacted in "stunned amazement, which quickly turned into uncontrolled euphoria." Nasser ordered a crackdown against Syrian communists, dismissing many of them from their governmental posts.
On 14 July 1958, Iraqi army officers Abdel Karim Qasim and Abdel Salam Aref overthrew the Iraqi monarchy and, the next day, Iraqi prime minister and Nasser's chief Arab antagonist, Nuri al-Said, was killed. The entire Iraqi royal family was killed, and Al-Said's and Iraqi crown prince 'Abd al-Ilah's bodies were mutilated and dragged across Baghdad. Nasser recognized the new government and stated that "any attack on Iraq was tantamount to an attack on the UAR". On 15 July, US marines landed in Lebanon, and British special forces in Jordan, upon the request of those countries' governments to prevent them from falling to pro-Nasser forces. Nasser felt that the revolution in Iraq left the road for pan-Arab unity unblocked. On 19 July, for the first time, he declared that he was opting for full Arab union, although he had no plan to merge Iraq with the UAR. While most members of the Iraqi Revolutionary Command Council (RCC) favored Iraqi-UAR unity, Qasim sought to keep Iraq independent and resented Nasser's large popular base in the country.
In the fall of 1958, Nasser formed a tripartite committee consisting of Zakaria Mohieddin, al-Hawrani, and Salah Bitar to oversee developments in Syria. By moving the latter two, who were Ba'athists, to Cairo, he neutralized important political figures who had their own ideas about how Syria should be run. He put Syria under Sarraj, who effectively reduced the province to a police state by imprisoning and exiling landholders who objected to the introduction of Egyptian agricultural reform in Syria, as well as communists. Following the Lebanese election of Fuad Chehab in September 1958, relations between Lebanon and the UAR improved considerably. On 25 March 1959, Chehab and Nasser met at the Lebanese–Syrian border and compromised on an end to the Lebanese crisis.
Nasser played a significant part in the strengthening of African solidarity in the late 1950s and early 1960s, although his continental leadership role had increasingly passed to Algeria since 1962. During this period, Nasser made Egypt a refuge for anti-colonial leaders from several African countries and allowed the broadcast of anti-colonial propaganda from Cairo. Beginning in 1958, Nasser had a key role in the discussions among African leaders that led to the establishment of the Organisation of African Unity (OAU) in 1963.
To varying degrees, Nasser's statist system of government was continued in Egypt and emulated by virtually all Arab republics, namely Algeria, Syria, Iraq, Tunisia, Yemen, Sudan, and Libya. Ahmed Ben Bella, Algeria's first president, was a staunch Nasserist. Abdullah al-Sallal drove out the king of North Yemen in the name of Nasser's pan-Arabism. Other coups influenced by Nasser included those that occurred in Iraq in July 1958 and Syria in 1963. Muammar Gaddafi, who overthrew the Libyan monarchy in 1969, considered Nasser his hero and sought to succeed him as "leader of the Arabs". Also in 1969, Colonel Gaafar Nimeiry, a supporter of Nasser, took power in Sudan. The Arab Nationalist Movement (ANM) helped spread Nasser's pan-Arabist ideas throughout the Arab world, particularly among the Palestinians, Syrians, and Lebanese, and in South Yemen, the Persian Gulf, and Iraq. While many regional heads of state tried to emulate Nasser, Podeh opined that the "parochialism" of successive Arab leaders "transformed imitation [of Nasser] into parody".