"The Ottoman decline thesis" is a term we use to refer to a paradigm in Ottoman history that was predominant for most of the twentieth century. The decline thesis divided Ottoman history into two broad eras: a "rise" in which the empire's borders were expanding, its institutions were functioning smoothly, and its leadership was capable, and a "decline" in which the empire's borders were shrinking, its institutions were decaying, and its leadership was incompetent. The period in which the empire began shifting from one state to the other was usually placed around the death of Süleyman the Magnificent in 1566. Not all historians approached the empire's history in the same way, but this general view became very widespread as a basic framework for Ottoman history as a whole. The idea was, in effect, that the empire "worked" properly during the first half of its history, and was dysfunctional during the second half of its history, the occasional strong leader aside. The Tanzimat and its accompanying reforms during the nineteenth century were interpreted as an ultimately unsuccessful effort to stop this inexorable decline.

The earliest critiques of the decline thesis emerged in the 1970s and picked up steam in the '80s and '90s. These critiques came from two different directions. First, the theoretical direction: the idea of "decline" was distorting the way we understood Ottoman history. It was causing us to view its history in light of events that hadn't happened yet, as if the outcome was inevitable - thus the events and processes of the seventeenth and eighteenth centuries were being interpreted in relation not to the actual context of the time and place in which they occurred, but as "seeds of decline" that supposedly caused the fall of the empire hundreds of years later. It was also causing us to inappropriately idealize the so-called "golden age" of the empire - the period right before the decline supposedly set in - as some kind of perfect era in which the empire was functioning at peak efficiency. This idealization of the golden age further distorted our interpretation of later periods because it caused us to understand all change as negative, to see the changing nature of the empire's institutions as the "decay" of systems that had previously worked properly but which were now undergoing decline.

In effect, the decline thesis paradigm placed the Ottoman Empire outside of normal history. Normal states don't "rise and fall" in some kind of set pattern, they just... exist. They react to historical processes and adapt to them over time. This is where a lot of confusion comes from regarding what exactly "decline" means. "The decline thesis" was a paradigm, whereby the very story of Ottoman history itself was that of this rise-and-decline. "The Ottoman Empire was in decline" did not simply mean, as is sometimes imagined, "the Ottoman Empire's military strength and economic vitality decreased in relation to its neighbors." Rather, it meant that every aspect of the empire's history was conceptualized within this framework. It's not that the empire just got weaker, but that the nature of the empire and its place in world history centered around this idea of decline and decay. Nobody wanted to talk about, for example, the growth of international trade and economic boom of the eighteenth century, except to connect it to later European dominance of the Ottoman economy. The decline thesis proposed that all that mattered about Ottoman history in its later centuries was the story of how the empire became weaker and ultimately vanished. Nowadays, we want to write a history that actually tells us what was going on in the empire at the time. Not just, "how did the growth of international trade contribute to the empire's later weakness?" but "how did the growth of international trade impact contemporary Ottoman society?"

The second angle from which the decline thesis was attacked was more practical. When historians began to more critically engage with the Ottoman source base from the seventeenth, eighteenth, and nineteenth centuries, they found that a lot of our old beliefs about the empire's weakness, conservatism, and corruption were overblown. The condition of the empire really wasn't as negative as had been portrayed, and there wasn't some kind of continuous downward trend. Economic and fiscal crisis in the seventeenth century was followed by eighteenth century efflorescence and budget surpluses. The Ottoman army didn't just collapse into a chaotic mess after the sixteenth century, but continued adapting to the changing circumstances of war in the seventeenth century. Seventeenth and eighteenth century rulers weren't incompetent just because they didn't all imitate the martial traditions of previous eras. The list of piecemeal revisions to our understanding of the practical realities of these later centuries is very long, but the general trend has been to overturn the image of the Ottoman Empire as a state in decay. The various transformations that the empire experienced over the course of its long history need to be understood in light of their own particular contexts, not in light of an outcome - the fall of the empire - that was in many cases hundreds of years in the future.

There's a fascinating historiography question here.

In some ways, you can't deny the decline of the Ottoman Empire. The size of the Empire in 1600 was larger than it was in 1800, which was larger than it was in 1839 and then 1880 and then 1900 and then 1914. It gets progressively smaller over time, and this decline is why we still have this notion of the Ottoman Empire being "the sick man of Europe" - an Empire terminally ill, fated to wither and die.

The reason that many modern histories reject this thesis is that territorial contraction masks all the developments that did occur in the Ottoman Empire in the centuries after the 16th century. In military terms, the Ottoman Empire was still a powerful force that was able to lay siege to Vienna in 1683 and whose formal borders still extended almost as far as Austria into the 20th century. In the early 1800s, reformers like Selim III identified the Janissary corps as something that needed replacing with a modern army, the Nizam-ı Cedid. Then with the Vaka-yı Hayriye in the 1820s, Mahmud II forcefully disbanded the Janissaries once and for all. In other words, the Ottomans saw problems and took steps to address them.

The nineteenth century saw many other attempts at reform, most famously the entire Tanzimat era (both tanzimat and nizam share the same linguistic root of "reordering" incidentally), which saw changes on many fronts, ranging from public works in Iraq to a modernisation of the financial system to the state's printing of regional newspapers in local languages. 1878 and 1908 saw constitutions, elections and the development of modern concepts of Ottoman citizenship. It might have all come crashing down shortly thereafter but this wasn't obvious or inevitable.

So back to decline for a concluding thought: there's space for thinking of Ottoman decline being relative (as in relative to the European powers) but not absolute. Rejecting a simple "decline thesis" isn't about showing Ottoman accomplishments or successes, but rather reminding us that it wasn't a static, unchanging place that could never have kept up with its European neighbours.