What factor played a major role in the decline of the ottoman empire?

The Ottoman Empire declined because it allied with Germany during World War I.

Explanation:

The decline of the Ottoman Empire occurred from its division in the first decades of the twentieth century, which occurred mainly by the First World War.

As we know, the Ottoman Empire was one of the largest in territorial terms and one of the longest in history. Between the fifteenth and eighteenth centuries, lived its great apogee, with a vast region under its yoke.

Having contracted an aversion against the Russian Empire over the course of several decades when World War I broke out, the Ottoman Empire positioned itself alongside the Germans (with whom they also had trade agreements), the Italians and Austrians and the allies of Russia, that is, the United Kingdom and France.  

The war actually ended in 1918 with a widespread catastrophe on the European continent and a humiliating defeat of Germany and its allies, leading to the decline of the Ottoman Empire.

a is correct

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which statement explains one of the reasons world war 1 was so much more destructive than earlier conflicts