Some comments to an earlier post talk about who would win in a war between various Middle Eastern countries. There's a lot to be said for economic and military technological analysis, but looking over the broad sweep of history shows that strongly united people conquer those who are internally fragmented.
The Mongols are the best known example -- economically and technologically primitive compared to the many large-scale agrarian civilizations that they crushed with little effort. But after awhile they, too, got crushed by some of those civilizations, showing that it was not just some magical "X factor" that the Mongols had discovered.
They were under pressure from all sorts of sedentary empires, which forced them over the long term to band together. As they conquered the civilizations, the Mongols lost the original motivation to stay strongly united -- they were now the ones resting on their laurels. At that point, they lost the glue of solidarity, and were easily conquered by other groups who had grown strongly united against the Mongol menace.
Peter Turchin discusses this view of ethnogenesis, or how an ethnic group is born (and dies), in the pop audience book War and Peace and War. The process is more intense when the enemy closing in around you is foreign in a number of crucial ways, rather than similar to you. For example, they speak a language from an entirely different family, worship different gods, follow different food taboos, practice a different subsistence mode (nomadic pastoralist vs. sedentary farmer), and so on.
That basic framework can be applied to the contemporary Middle East to assess who is relatively more unified and who is relatively more fragmented. That will make big predictions for who would win in a war. So let's have a look at ethnic frictions within a country, especially if they rise to the degree of ethnic separatism.
By far the strongest nations on that score are Iran and Egypt.
While Egypt does have a strong internal conflict between rural and urban, traditional and modern, like any country, both sides in those conflicts still see themselves as "Egyptian" rather than some non-Egyptian group. Even the Coptic Christian minority does not have breakaway grievances, despite occasionally getting blown up by jihadists as they were last Easter.
Iran is far more diverse, so its absence of separatism is an even stronger testament to its national unity. Only 60% are ethnic Persians, who are Shia Muslims. Another 15% are Azerbaijani, who are also Shia, but who speak a non-Indo-European language unlike the Persians. It's unclear from my cursory look at the genetic findings whether these Azerbaijanis are genetically similar to the Persians and have adopted a foreign group's language, or whether that foreign language correlates with a foreign genepool. In any case, they are a distinct group, whether they feel the basis to be genetic or cultural.
The Kurds, who are mostly Sunni Muslims, make up another 10%, and they are closer to the Persians in genes and language, being more like sisters or cousins, although they tend to be more nomadic than the sedentary Persians, and that's no small difference. At 5%, the Lurs are similar to the Kurds -- a nomadic group speaking a similar language to the Persians -- but like the Persians, they are generally Shia Muslims.
That leaves fully 10% who are still not like any of these other groups -- Turkic groups like the Turkmens, Caucasian groups like the Armenians, (Middle Eastern) Jews, and others.
The Azerbaijanis are more or less completely assimilated into the Iranian nation, and indeed founded a dynasty (the Qajars) that ruled Persia from 1785 to 1925. The Grand Ayatollah Khamenei (head of state and Commander-in-Chief of the armed forces), is also of Azeri background.
The smaller ethnic groups are even alloted a quota of seats in the Parliament: 2 for Armenians (Caucasian, Christian), 1 for Assyrians (Semitic, Christian), 1 for Jews (Semitic, Jewish), and 1 for Zoroastrians (Persian/Iranian, pre-Islamic religion).
That leaves the Kurds as the main risk. Unlike elsewhere, they have mostly given up on separating from their nation of residence. The last major Kurdish rebellion was during the infancy of the Iranian Revolution in the late '70s and early '80s, when the new nation was inchoate and vulnerable. They are not as brutally suppressed as they are in Turkey.
Since Kurdish separatism in Iran has sharply declined over the decades, the trend in Iran is toward stronger national unity.
So where does that leave the other countries? Here are the ethnic divisions in major countries that risk fragmenting national unity during the crucial event of war:
Saudi Arabia has a 15% Shia population under Salafi rule, or Sunni extremists (the same who spread jihadism through radical mosques and paramilitaries like al-Qaeda and ISIS). Worse, this minority is at high concentrations in the oil-rich areas along the Persian Gulf, which over the millennia has fallen more under the cultural and ethnic influence of Eastern Mesopotamia and Persia rather than the Arabian Desert where the current regime is based (Riyadh). There's also a decent cluster of Shia in the Hejaz region along the Red Sea coast, also outside of the Desert's cultural sphere, including the area of Medina (2nd holiest city in Islam).
Even worse for Saudi Arabia is that 1/3 of its population is foreigners, amounting to some 10 million people, mostly from South Asian or Southeast Asian countries with large Muslim populations. They are treated like second-class citizens at best. Imagine one-third of the population being immediately ruled out when it comes to being motivated enough to sacrifice one's life for the greater good of the nation -- it's not even their nation!
Israel of course has to worry about the Palestinians, who make up roughly 50% of the population under Israeli control (Israel and the Occupied Territories of the West Bank and Gaza), and 20% of Israel proper. It's hard to fight both an internal and an external war because "When the cat's away, the mice will play." There are also less substantial ethnic fault-lines among the Jews themselves, between Ashkenazi and Mizrahi / Sephardi, although those do not reach the level of separatism.
Turkey has about 30% of its population in minority ethnic groups, with 15% being Kurds. Conflict between Kurds and the Turkish government has been armed and increasing over time, continuing right through the Syrian War, even when the Kurds are in another country. Kurdish separatism will be a major obstacle for Turkey to overcome in a war against a separate peer country like, e.g., Iran. Plus the Kurdish part of the country lies in the Southeast, forming a barrier between Istanbul or Ankara and most of the Middle East.
Syria is not a regional power, but it's worth considering just to show how ethnic diversity can weaken a nation. It too has a decent Kurdish population, although they are not as bitterly separatist as in Turkey and are not suppressed as brutally. Still, it's something they are organizing for, and something that antagonizes the non-Kurdish Syrians. The main foreign presence is Sunni extremists, and their jihadism is aimed at anyone who isn't Sunni -- of which Syria offers many targets, from Shia to Druze to Alawite to Christian. If these Gulf-derived jihadists radicalize the local Syrian Sunnis -- to make them view religious sect rather than national origin as primary -- then they can seriously weaken the state, as we've seen during the Syrian Civil War.
If the jihadist presence continued over the long-term, and especially if it were associated with foreign nationals rather than local Syrian Sunnis, that would strengthen the Syrian national identity, as the diverse groups band together against the jihadists. As it stands, though, Russia has intervened to save them from prolonged exposure to the jihadists. That will probably leave the Syrians breathing a sigh of relief, and going back to their not-so-intense level of national solidarity, rather than becoming so intensely nationalistic that they invade the Arabian Desert and wipe out their nemesis.
That still leaves the superpowers who are in the region.
Russia has come more together as a nation after Putin came to power (or his election was the initial expression of rising national solidarity). That was after the Chechen wars of the 1990s, not to mention the numerous non-European nations breaking away from the Soviet Union. That removed a lot of the ethnic diversity problem. On the other hand, they have added a big chunk of ethnic Russians from the historical land of the Russian Empire, namely Crimea.
The United States, as we know, could not be more ethnically fragmented unless the Devil himself intervened. It has not reached armed separatism like the Chechens did in the 1990s against Russia, but the various Hispanic groups who do not consider themselves American and want to break away in some way, while still getting the goodies from the American government, is a big stumbling block. (Solution: deport illegals, anchor babies, and their families.) And now that Obama is no longer President, blacks don't feel as American as they used to.
The Muslim population is the most openly revolting, by refusing to assimilate to even a small degree, and by being so over-represented among spectacle-style terrorism against the American nation. White mass murder is infinitely less common per capita, and is not so strictly focused at the American government, people, or culture as a whole (Oklahoma City bombings are the exception, whereas most are "going postal" incidents with a more local focus).
There are deep divisions within the white population, but those are not at the level of ethnic conflict or separatism. They are the blue state vs. red state, liberal vs. conservative, urban vs. rural, modern vs. traditional divide that we see in many countries without ethnic separatism. Still, it has become more common for whites to identify as hyphenated Americans rather than just Americans, and in some cases they don't even use the hyphen -- many Irish-"Americans" refer to themselves as simply "the Irish" just like "the Jews" do to themselves.
Relatively speaking, then, Russia is the more nationally unified superpower.
Looking at the major alliances in the Middle East, we can predict that the Russia-Iran side has the advantage over the Saudi-Israeli-Turkish side, when it comes to acting with a collective unity of purpose.
The big wild card is Egypt, which used to be squarely on the Saudi-Israeli-Turkish side but has begun to seek rapprochement with Iran. The litmus test right now is whether you favor regime change in Syria or not, and al-Sisi is decidedly in the "no" camp, along with Iran and Russia. Russia is also going to be building a nuclear power reactor in Egypt, no small favor. Egypt has ordered 50 MiG-29 fighters, although I don't know enough to say whether they would switch over to relying on Russian rather than American suppliers for their military.
At the very least, al-Sisi may try to revive Nasser's stance of relying equally on the US and Russia. That means in any given battle or war, Egypt may be fighting on the Russian-Iranian side. If it did, that would really re-arrange the map, with peripheral nations penning in the core nations -- Russia coming in from the North, Iran from the East, and Egypt from the South and West. That lies well into the near-to-medium term, though, but it does seem to be moving in that direction rather than staunchly on the American-Saudi side.