Found the following article online, use it to answer the question: Which war resulted in significant territorial losses and a total of 87,500 soldiers killed?  The Treaty of San Stefano was signed on 3 March 1878 by Russia and the Ottoman Empire. It was to set up an autonomous Bulgarian principality spanning Moesia, Macedonia and Thrace, roughly on the territories of the Second Bulgarian Empire, and this day is now a public holiday called National Liberation Day. The other Great Powers immediately rejected the treaty out of fear that such a large country in the Balkans might threaten their interests. It was superseded by the Treaty of Berlin, signed on 13 July, which provided for a much smaller state only comprising Moesia and the region of Sofia, leaving large populations of ethnic Bulgarians outside the new country. This significantly contributed to Bulgaria's militaristic foreign affairs approach during the first half of the 20th century.The Bulgarian principality won a war against Serbia and incorporated the semi-autonomous Ottoman territory of Eastern Rumelia in 1885, proclaiming itself an independent state on 5 October 1908. In the years following independence, Bulgaria increasingly militarized and was often referred to as "the Balkan Prussia". It became involved in three consecutive conflicts between 1912 and 1918—two Balkan Wars and World War I. After a disastrous defeat in the Second Balkan War, Bulgaria again found itself fighting on the losing side as a result of its alliance with the Central Powers in World War I. Despite fielding more than a quarter of its population in a 1,200,000-strong army and achieving several decisive victories at Doiran and Monastir, the country capitulated in 1918. The war resulted in significant territorial losses and a total of 87,500 soldiers killed. More than 253,000 refugees from the lost territories immigrated to Bulgaria from 1912 to 1929, placing additional strain on the already ruined national economy.
The answer to this question is:
World War I