The time between 1915 and 1917 was pan-Arabism's greatest chance for success, but British and French desires to maintain the Middle East in broken down pieces for easy division and British belief that the Sharif could not control a large state prevented any real agreement with the Sharif on the makeup of the post-Ottoman Arab state. When the Sharif realized the British had betrayed him with the Sykes-Picot Agreement, his family took what it could: control in Jordan and Iraq. Of course, the British had more than an observational role in the Sharif's ultimate collapse and ouster in Saudi Arabia, supporting his rival Ibn Saud, who took over all of Saudi Arabia by 1932. With the Sharif's failure combined with the British and French imposing minority governments on Jordan, Iraq and Syria, the Pan-Arabist ideology flickered out. Abdullah of Jordan also sought to put himself as the leader of a pan-Arabist "Greater Syria," but his assassination by a Palestinian nationalist in 1957 put all such plans on the back burner.
In reality, Palestinian nationalism's failure between the 1940s and Six Day War came about in part because it was one of the only national movements that focused on separating itself from its neighbors while King Abdullah and eventually Gamal Abdel Nasser were focused on forging a unified Arab State in the mold of the Sharif of Mecca's plan from World War I. Pan Arabism and its leaders faced their ultimate test in 1967. Nasser, increasingly belligerent and imagining himself as a modern day Saladin used a lot of bluster and a blockade of the Straits of Tiran to escalate conflict with Israel. Finding support from a receptive government in Moscow and emboldened by his own grandiosity, Nasser was caught completely unprepared for Operation Focus, in which the IAF destroyed the vast majority of Egypt's air force on June 5, 1967. With the catastrophic collapse of Arab armies and the expansion of Israel into the Sinai, Judea, Samaria, Gaza and the Golan Heights, Pan-Arabism became associated with Arab failure to displace Israel.
Although the Yom Kippur War brought back a measure of prestige to the Egyptians, by then Nasser was dead and pan-Arabism was on the wane. Saudi Arabia became a greater power in the region by virtue of Egypt's weakened state and perception that Anwar Sadat and Hosni Mubarak were too moderate and unwilling to champion the cause. In its own way, Israel became a symbol of the West's supposed unwillingness to let go of the Middle East and its continued attempt to manipulate and control the flow of oil while also maintaining the upper hand in the ideological battle.
The reborn Palestinian nationalism, in the form of the PLO, came of age in this primarily post-Arabist world. Because leaders in Jordan, Lebanon and Syria were focused of forging pan-Arabism, led by themselves of course, and were still largely minority and/or foreign governments (Hashemites in Jordan, Alawites in Syria and Christians in Lebanon), the PLO embraced increasingly turned away from cooperation with those governments against Israel and moved toward the rising Islamist movement. In 1979, the Islamic Revolution in Iran gave Islamism its strongest voice and a strongly-anti Western message. Iran's revolutionary government rightfully saw the Pahlavi Shah Mohammed Reza as an American and British puppet put back in power after Mohammed Mossadegh attempted to nationalize Britain's Anglo-Iranian Oil Company (later known as British Petroleum). Iran became the flag-bearer for anti-Western passions by virtue of its own history with Britain and America and came up at a time when the PLO was seeking a significant international patron. Unlike the Egyptians and Syrians, Iran had no obvious goal to take over Palestine and as non-Arabs, they represented little direct threat to the Palestinians. Iran was, of course, embroiled in an eight year war against Saddam Hussein's Iraq. When the U.S. toppled Saddam in 2003, it removed Iran's most powerful counterbalance, empowering the Iranian government to enflame the passions of Iraq's Shi'ite minority and to push itself toward leadership of the Muslim world as Saudi Arabia became viewed as more and more connected to America, and thus, untrustworthy.
Pan-Arabism is dead and secular nationalism lags far behind Islamism as the dominant political ideology in the Middle East. In many ways, Nasser's failure in 1967 sounded the death knell for pan-Arabism and to a greater extent Arab nationalism as the dominant ideology for opposing the West. Islamism arrival in Tehran in 1979 presented another option. Both ideologies were focused on a singular goal: removing the stain of Western influence on the Middle East. Pan-Arabism was a response to the post-World War I border drawing, under which Arab states were created based on Britain and France's agreed upon spheres of influence with no regard for historical tribal distribution. The idea of creating a unified Arab state that hearkened back to the Umayyad or even Mamluk empires was a way to restore the Arab world to prestige. Of course, deciding who would lead that Arab state proved to be among the ultimate failings of pan-Arabism. Likewise, despite Islamism popular appeal, theological differences have caused fragmentation among the centers of Islamic power. Iranian Shi'ites, who finally liberated themselves from a millennium of Sunni oppression, have little love for the Saudi Wahhabis. Meaningfully, there seems to be little desire to return the dormant Caliphate as there is a desire for each Islamic country to purify itself from Western influence by looking to its own Islamic tradition, be it Salafi in Egypt, Wahhabi in Arabia or Shi'ite in Iran.
Both of these ideologies focus on Israel, which is perceived as continuation of Western meddling in the Middle East. Palestinian nationalists, seeing that other nations were adopting their cause as a way to strike at Israel, have turned from secular nationalists in the PLO to Sunni Islamism (and to some extent Hizbullah's Shi'ite Islamism) in Hamas. By ideologically aligning the Palestinian struggle with the ideology of the dominant political forces of the region, the PLO continues to wane in influence while Islamists in Hamas and Hizbullah assume greater power. Of course, part of the shift toward a more politically radical ideology entails greater violence and an ultimate unwillingness to negotiate or acknowledge Jewish nationalism in the Middle East. While the PLO could at least lie about accepting Israel, Hamas, Iran and Hizbullah do not and will not disguise their uncompromising desire to bring about the end of Jewish sovereignty in Israel and make sure that the Jews do not rise from their dhimmi status ever again.