Prior to 1683 and the Battle of Vienna, the Ottoman Empire was at its height in Europe. It controlled all of Thrace (present-day Bulgaria and European Turkey) as well as nearly all of the Balkans today (Romania, Greece, much of former Yugoslavia, etc.).

In the Battle of Vienna, the last wall defending Christendom (the Austrian Hapsburgs in control of the Holy Roman Empire) decisively defeated the Ottoman armies and subsequently invaded Ottoman-controlled Hungary and the Balkans. The Great Turkish War as it is known ended in 1699 with the "Holy League" defeating the Ottomans, reclaiming Hungary and the principality of Transylvania. You can see a before and after map here (before) and here (after).

Following this defeat, the Ottoman Empire would see its former empire crumble over the next two hundred years. Constant warfare with the Russian Empire (which saw mostly Ottoman defeats) as well the numerous revolutions in the 1800s deterred the Ottomans from holding any real power in the region. Greece gained independence in 1829-30 following assistance by the British, French, and Russians at the Battle of Navarino. Romania, Serbia, and Montenegro gained independence in the Treaty of San Stefano after another Ottoman defeat at the hands of the Russian Empire. Bulgaria later became semi-independent and a fully independent kingdom in 1908, the same year that Austria officially annexed Bosnia-Herzegovina (which it had occupied militarily since 1878). The Balkan Wars occurred in 1912-3 which saw the near end of all Ottoman territory in Europe, save for the same East Thracian province encompassing present-day Edirne, Turkey. World War I (as you know) was the final nail in the coffin for the empire.

An important thing to mention is how did this eventually happen. For starters, the Ottoman Empire started suffering a decline in the 1700s and 1800s based on internal strife and corruption. As is the case for all empires that get too large and have many different ethnicities within its borders, it's easy to deduce how the Empire struggled so much in international circles.

This idea of different ethnicities is a prime example of its troubles in Europe. Throughout history, the Ottomans controlled Bosnians, Greeks, Serbs, Albanians, Bulgarians, Romanians, Hungarians, Germans, Croats, and many other ethnic and national groups. The rise of nationalism as the idea of the modern state developed made it much more difficult for large, multi-national empires like the Ottomans to keep its vast holdings together.

Another factor to point out was the Ottoman relationship with the other two large empires in Eastern Europe (the Austrian Empire, later Austria-Hungary, and the Russian Empire). Russia always had a desire to control the Black Sea's entrance to the Mediterranean, called the Dardanelles. As this was controlled by the Ottomans, this lead to frequent conflict between the two empires. Furthermore, the Russians felt compelled to aid their Slavic brothers in the Balkans against Ottoman tyranny. This is why Russia mobilized following Austria-Hungary's declaration of war on Serbia in 1914. The Austrians, meanwhile, were fearful of the Russians because they felt threated by a large Slavic state allied with Russia on its Southern borders. Both empires basically chose to use the Ottoman Empire (by this point known as "the sick old man of Europe") to either advance its ambitions or secure its borders and interests.

Sorry for this giant wall of text. If you have any more questions, I'll be happy to answer.