I'd argue that Ottoman "period of national greatness" was 1320-1570. After that, the bureucratic and military structure of the central government was just managing the decline. I read this very good paper by Glubb Paşa (Sir John Glubb); Fate of the Empires. It was about 25 pages but it really broadened my horizons.Basically, there are patterns to 'empires'. From Ibn Khaldun to Glubb Pasha to Oswald Spengler, people just recognize that pattern.
Khaldun would argue about some kind of primal invaders because he lived in middle ages in the Islamic world. There was a stark contrast between the decidedly urban and tame muslim city folk and the invaders, who were basically mercenary barbarians. There is also a strange parallel with what Turks did to the muslim world and what Germanics did to the western christian world. They both took power by way of invasions and such, and became the "ruling elite" of the respective civilizational blocs. Khaldun was an acute observer of this strange phenomenon, and coined the term 'assabiyyah' for the seemingly unending source of action and energy that these barbarians posessed. Assabiyyah is what drives groups people to take action, perservere in the face of adversity and work towards a common goal as a united front.
Glubb pasha was more analytical about it, and he had the more scientific history to depend on to draw conclusions, plus his lived experience as a subject of the British crown in the early 20th century. He even listed the characteristics of rising and dissolving empires. He also defined what an empire is. Glubb Pasha says that an empire is; a center of power that dominates its 'world' - how big that world is depends on technological level mostly. Assyrians can dominate the fertile crescent and eastern Mediterranean, British can project power through oceans, Americans can interfere in tribal conflicts in jungles.
The empire goes through stages of life. I'll give some examples.
- Age of Pioneers: extraordinary displays of courage and action. Action, action, action. Rulers often don't even try to justify their moves. Think of Mongol invasions. In Ottomans' case, taming of the Balkans and consolidating Anatolia, culminating in the Conquest of Konstantiniyye.
- Age of Conquests: Now, our nation goes on to conquer vast swathes of territory. Think what Selim the Grim did, and Suleiman's early rule.
- Age of Commerce: Conquests stop. Wars may continue, but whether it is geopolitical, technological or societal, some kind of constraint stops the expansion. Suleiman's later rule is a good example of this; from the messianic conqueror Khan into a kind of a father figure 'wise sultan'. Trade flourishes. Wealth is 'created' in this stage. Because of rapid economic expansion, exploitation is relatively less egregious than the later stages.
- Age of Affluence: People become gradually selfish. A lot of clout chasing, bootlicking, back-stabbing. Rentier class grow in size and influence. "Let them eat cake" mentality slowly begins as the ruling elite become disconnected with the peripheral elements of society such as the urban poor and the 'provincials'. People begin to think that sacrifice is a needless folly. Morals begins to transform into ethics. Honor is no longer something to die and kill for. Wealth is distributed in this period. You have money not because you make something of yourself, but because you steal from or defraud others by legal or illegal means.
- Age of Intellect: People debate things in and out of existence. While problems grow, no one takes action. The notion that intellect itself can solve all issues goes rampant. Things become sterilized, or at least they try to be.
- Age of Decadence: Crises reach into their respective breaking points. Whatever form they take, they just transform the imperial order into something that can not be described as the imperial order. In the Ottoman example it's corruption of devshirme system and Celalî rebellions transforming the classical Ottoman imperial order into a bureucratic mess that tries to hold on to what it got, utilizing its image of invincibility, while a million hungry mouths eat away its central life-force.
Oswald Spengler is by far the most wacky of the bunch, because he was kind of an esotericist and a dilettante. There is an awesome but old youtube channel by his name, you should check it out. Warning: the videos hit different if you are baked out of your mind.
Spengler's epochs go beyond specific empires. Dude talks about entire civilizations. He believes the civilization revolves around a central concept, and everything produced by the civilization reflects that. Western civilization, he says, have
infinity as its focal concept. Architecturally, it is gothic cathedrals for example. Then the civilization goes through life-stages of spring, summer, fall, winter. Usually it takes about 1000 years. I'll explain with an example; a bastardization of his theory can be applied to Turkish civilization in Anatolia.
- In the spring you have a decidedly rural population. Turkmen settlers who don't care, nay, don't even think about what the city-folk or the ruling class thinks unless it becomes a problem ("you can't get those pastures" kind of conflict). People are religious, but their religiosity doesn't have to be reflected in an organized religion. In our example, Turkmen settlers were wildly heterodox. But their belief in a higher power is not in question. They believe in it, and in some cases kill or die for it, but they are just pragmatic about it. The culture entrenches itself into the land, and puts down roots. Military action in this stage is kind of mythical, think Danişmend Ghazi and stuff. Spring-period people go hard on the Dede Korkut stuff.
- Then comes the summer period. It's the mid-day, where the sun shines the brightest, and few shades remain where the warming (or burning) rays of the sun doesn't reach. This period is characterized by institutionalization and things reach their zenith. Folk songs take the back seat against more 'urban' styles (less türkü and more Türk sanat Müziği). Less church music and more classical symphonies. The energy possessed by the civilization bursts into military expansionism. Think Ottoman conquests.
- Fall period comes with all the pleasant bur corroding characteristics that is described above in the age of affluence and intellect. The city dominates the rural in this period, think Celalî rebellions. The later stages described in Glubb Pasha's paper are also valid for Spengler, just for a larger period.
- Winter: The culture gradually dies. Another may rise from its ashes. Spengler's example is Roman culture dying to make way for the western civilization to be born. I firmly believe early republican idealism reflected the rise of a new culture. If we can purge the corroding elements, we can start anew but the world is a lot more complicated than a thousand years ago.