The prolonged contraction of the Ottoman empire supercharged problems with minorities. As Christian states gained independence and/or new territories there was a tendency for many of these states to expel or kill Muslims from previously mixed areas since some influential persons regarded them as a potential fifth column who may want to help the Ottomans to come back due to shared religion. So for instance Serbia enacted a policy to remove Muslims from the region of Nis after gaining control over it. As Muslims would flee the contracting borders over to where Ottomans still held sway, though, these refugees would be quite angry and vengeful towards local Christians who they would often view as potential traitors who could take their homes just like at their earlier homes, and they would tell awful tales of what happened to them that would radicalize local Muslims. The presence of the refugees would also tilt the ethnic balance towards their favor. This combination of being locally outnumbered by people angry at them could lead to riots and massacres, not a safe situation, so the Christian minorities would often themselves pick up and flee across that same border going the other way… and guess what? These Christian refugees had a very similar chip on their shoulder and reacted in similar ways when settling in areas that still had mixed populations! Once the ball got rolling the process of ethnic polarization could autonomously sustain itself without the governments involved needing to keep on supporting it since it was based on the rage of local people. Sometimes governments would lean into it for reasons like to exert more control over distant restless areas and sometimes governments would pump the brakes for reasons like wanting the taxes of the people locals were wanting to expel. Kind of depended on who would be in charge, the problems they were dealing with and how they prioritized them. Going back to the Serbia and Nis example, Serbia got an ethnically ‘clean’ Serbian Orthodox Nis, but in doing so the Muslim Albanians they kicked out set up in previously mixed Ottoman Kosovo and that combined with the accompanying outflows of Serbians decisively shifted the demographics on the ground in Kosovo to majority Albanian Muslim.

A good chunk of people in what is modern day Turkey, something like a quarter or third, are descended from the millions of Muslim refugees who fled areas like the Balkans and the Caucasus (where Russia was slaughtering Circassians); these refugees are called Muhacirs from an Arabic word for immigrant/emigrant. With so many having these sorts of traumatic experiences or knowing someone who did (and the leadership in Istanbul being exposed to many many many of these people given that Thrace is the area many of the Balkan Muslims went to), the late Ottoman Empire and Turkey developed a very strong sense of paranoia and suspicion towards the remaining Christians in the empire.

If you had to chalk it up to a foreign power I would say Russia was both the main reason for Ottoman control slipping in the Balkans and through its internal policies of expulsion and extermination was the source of over a million Muhacirs which was a big contribution to that process of ethnic polarization. But the French and the British were certainly happy to pile on with divide and rule strategies to inspire revolts when they found themselves on the opposite side of a war with the Ottomans, and to maintain such policies for ruling what they gained.

edit: while the above explains while relations between the Ottomans and Christians became fraught, I should also mention ones that frayed ties with groups that generally shared religion with the government also. As the Ottomans were attempting to reform they were trying to ape the modern nation state models of the time of the leading empire of the day. Those models really did not allow for the kind of linguistic diversity that the Ottoman Empire had, which led to the government trying to impose Turkish language generally. This is not that much of a problem for small groups of muhajirs spread diffusely in Anatolia where many of their neighbors were speaking Turkish anyway to accept; however, in areas that as a block were speaking Arabic and Kurdish and so on that was much more difficult pill to swallow that led to big problems. Plus these groups generally had much more respect for the Caliph than they did for the modernizing government with the Turkish nationalist CUP. As WWI carried on the British were able to convince many local Arab leaders to revolt with pretty grand promises of a huge Arabia (that they would not follow up on) and Turkey lost control of much of the Arabian areas. But there were still some decently strong Kurdish ties even in into the earliest history of the modern Turkish government since the Ottomans had cultivated those ties for hundreds of years as their first line of defense against the Qizilbash and Azeri Turks I guess you could say who were generally considered aligned with the Shia dynasties of Iran. But when Ataturk’s Turkey overcame the Ottoman government, there were problems that came because of the secularizing nature of the government. The old sultan had political power removed, went into exile and his successor was just left with the religious title of caliph. He asked for a raise in his stipend and some foreign scholars asked for him to have more power in the government. Ataturk being a very aggressively secular guy who did not at all value the caliphate seized on this as a chance to say the institution was a channel for foreign influence into Turkish politics and abolished the caliphate and expelled the caliph. This and other encroachments on religious institutions and culture was taken by many prominent Sunni Kurdish leaders including Sheikh Said as severing the last religious ties that they held in common and the Sheikh Said rebellion kicked off. It was intended that Muslims in general revolt but mostly only Kurds joined in and it took on kind of a nationalist character. Nevertheless they sieged Diyarbakir and it was very expensive to put down so Turkey did a report on it and the recommendations of that “Report for Reform in the East” introduced many policies aimed at cracking down on Kurdish society that caused many problems between them and the Turkish government.

No, Britain’s “Divide and rule” policy was largely implemented after WW1 and the dissolution of the Ottoman Empire, when Britain thought it could hold onto those regions themselves (de facto, if not de jure). Before that, the Brits (and French) actually went through pains to ensure that the Ottoman Empire didn’t fall apart, because it would damage the balance of power in Europe.

The crackdowns of the Ottoman Empire had two essential causes, basically relying on which period one is looking at. For most of the Empire, it was the same cause that every empire cracks down on regions and cultures for - insufficient loyalty to the central government. And very often, this was dependent on the relationship of central government officials with local magnates. Essentially clientistic relationships more than anything - questions of “You are taxing too much” or “You are offering too little.”

In the last few decades of the Ottoman Empire, both the rise of 19th century nationalism and the increasing realization of the Ottomans that the former despotic-clientistic structure could not compete with modern states caused the Ottomans to reform considerably. Unfortunately, these reforms, while nominally in-line with the ideals of the European Enlightenment and French Revolution, also were accompanied by notions of a Turkish nation, and increased distrust of minorities in the Ottoman Empire (who, to be fair, were themselves adopting 19th century notions of nationalism and getting ‘dangerous’ ideas about independence).

As the Ottoman Empire weakened, losing control over several minority Christian populations either to outright separatism (thus reducing the role and influence of Christians in the empire) or ‘safeguarding’ from European colonial empires claiming to be the ‘protector’ of Christians in a region (thus creating distrust towards Christians, even in the occasions when the safeguarding was legitimate), the Muslim character (always strong, mind you) of the Ottoman Empire intensified even as the formal legal position of Islam became less supported.

So in the late Ottoman Empire, you have a multiethnic, multifaith empire that essentially has adopted notions of being a Turkish, Muslim empire, and all the… unrest that comes with that level of cultural-religious chauvinism.

I don’t know if “permitting different cultures” is how I’d phrase/frame it.

There always existed different cultures in the region and the Ottomans knew that. So they didn’t outlaw any culture or religion, but applied a dhimmi status - something nationalistic identities today try to misconceptualize - which actually translates to ‘protected persons’ who paid taxes to benefit from the Ottoman Muslim state protection and governance (not to be confused with full equality though). If you were happy to be under the Ottoman empire and pay tax, then you’re a part of it.

The primary influencers from Europe were Britain and France (they carved up the ottoman empire post WW1). They definitely had a hand in applying ‘soft power’ through minority groups such as those wanting more autonomy or who were disenfranchised by the Ottoman empire’s sub-par reforms and modernization (like pug mentioned).

But it wasn’t really one thing/person/imperial’s fault.

The Ottomans often benefited from limiting the development of competing political identities because maintaining imperial cohesion was important to the survival of a multi-ethnic empire. At the same time, there were competing factions within the Ottoman political establishment, each with different ideas about how the empire should be preserved and governed. So there were proponents who wanted to oppress, and others who didn’t.

The Europeans benefitted by carving it up because that was their colonial model (tried and proven in Africa and Asia). Whatever influence they exerted was generally part of the normal great-power competition of the era rather than direct control, and not necessarily of a kind that forced the Ottomans to respond with repression.