Arab nationalism was one of the major political movements of the 20th century in the Middle East. Nationalism as whole was spreading throughout the Ottoman lands in the 19th century. It started with the Greeks and Serbians and spread essentially West to East. By mid century, Turkish nationalism was emerging, and by the late 19th century you start seeing more powerful Armenian and Arab nationalism (Kurdish nationalism doesn't really appear really very strongly until the 20th century). I'm visiting a friend who's primping right now in the bathroom so I don't have access to my books, but there's this great section in Geoffrey Lewis's Catastrophic Success about a Turkish bureaucrat remembering his school days in Aleppo during the very late Ottoman Empire when all the Arab students made fun of the Ottoman language for being heavily derived from Arabic and not really Turkish--how the Turks really added nothing of civilization to Ottoman culture. The man recalls this moment of Arab national identity as something that turned him into a strong believer in Turkish nationalism.

So Arab national identity starts to develop in the 19th century, but we don't really see Arab political movements coordinating across space until much much later. While people often credit the Arab Revolt during World War I as the start of Arab nationalism, one should realize that this was a very local event in the Hejaz area that barely made it into the Levant. Similarly, the Great Syrian Revolt is cited as another great moment of Arab nationalism but this wasn't even general over Syria, never mind the Arab World as a whole. Kurdish nationalism of this period incidentally had the same problem--people wanted to speak for an identity (or have local autonomy of some kind), but couldn't over come existing internal cleavages and effectively unite.

In the 30's Arab nationalism as a discrete political movement (rather than as an available translocal identity) starts to develop. It's especially popular with the western educated middle class who make up most of the bureaucrats--called the effendiyya class in much of the Arab World (Michael Eppel has a decent article on the effendiyya in Iraq of this period). The large land owners tend not to be supportive (as it would potentially challenge their power--Beruit based landlords are selling land in Palestine to the Zionists for much of this period, much to the chagrin of the other classes in the area). In some places, peasants develop a "national conscience" (Palestine for example) but in many other places this won't happen until later.

So by the 40's and the 50's Arab nationalism is one of the big movements in the region (Muslim Brotherhood and the Communist Party are the other big ones--MB in Egypt especially and the CP in Iraq especially). Nationalism is Arab nationalism (I always find it telling that Betty Anderson's Nationalist Voices in Jordan takes it for granted that the nationalism would be transnational Arab nationalism, rather than a more specifically Arab nationalism).

In the 40's, the Baathist starting out of the Levant become the strongest political movement for Arab nationalism. They mix nationalism with socialism, and are very inclusive of both Muslims and Christians. The Baathist eventually become dominant in Syria and then Iraq but are never able to unite. In Egypt, Nasserism arises as a major challenge to Baathism within the Arab nationalist movement. For a brief period, Syria and Egypt unite (at Syria's request) but this goes very badly for Syria, as it essentially means Egyptian gain influence in Syria without Syrians correspondingly gaining power in Egypt. Within two years, a coup in Syria ends this experiment in Arab unity.

After this and through the 60's Arab nationalism changes from Arab unity to Arab solidarity (Adeed Dawisha wrote the book on this). Ba'athist parties become more and more focused on national politics, rather than being a movement meant to coordinate internationally. The parties still espoused many of the same policies and promises--militarism, Arab solidarity, republicanism, socialism, ties with the Soviet Union, import-substitution industrialization, nationalization of industries, strong state hand in a planned economy--but in Iraq and Syria (the places where the parties become dominant) they also became tools for minority rule.

It's understandable of course how Arab unity might appeal to minorities. Arab Christians were important members of the Baathist parties since the beginning (and continued to be important in Iraq and Syria, though Christians were never as important in Nasserist Egypt). In Syria under Hafez al Assad, an Alawite in a Sunni majority country, the party became a network that was dominated by Alawites (though it wasn't exclusively Alawite). Similarly, Baathist Iraq especially under Saddam was a way for Sunni-dominated networks of party loyalist to rule a Shi'a majority country. Of course, that wasn't the design the Baathists but it's note worthy that autocratic minority rule was the result in both countries.

When the Soviet Union collapsed, both lost their major patrons. They had to start reforming the economy. As socialism became less convincing and Arab (inter)nationalism was a spent force politically, both countries turned to religion as a new unifier in the 90's. Both countries end up "upgrading their authoritarianism" (see [Steve Heydemann's excellent excellent paper for Brookings on what that means--it's freely available online).

But the rise and fall of Baathism falls larger patterns of Arab nationalism and Arab republicanism.