Imperial War Museum
Eight-inch howitzers of the 39th Siege Battery, Royal Garrison Artillery firing in the Fricourt-Mametz Valley during the Battle of the Somme, 1916.
The US joined World War I three years after it started, but there were few parts of the world that weren't touched by the Great War
The war started on July 28, 1914, just a month after Austro-Hungarian Archduke Franz Ferdinand and his wife were gunned down on the streets of Sarajevo, Bosnia, by a Serbian nationalist.
The assassination, and the subsequent political and military upheavals, led to the start of World War I, which would eventually roil much of the globe.
Less than three bloody years later, the US would be drawn into the conflict on the side of the Allies, declaring war on the Central Powers on April 6, 1917.
It is hard to comprehend the breadth World War I.
Though it was sparked by regional tensions, a web of entangling defensive alliances quickly pulled in almost the entire world and hastened the end of the European empires.
In May 2014, Reddit user Srirachachacha shared this map showing how the conflict spread across the world while distinguishing between Allied and Central Powers and their colonies, dominions, and territories.
By the time the war ended, the Russian, Ottoman, Austro-Hungarian, and Prussian Empires had collapsed. The war claimed over 37 million lives, and large swathes of Europe were in ruin.
Only two decades later - with the memories of World War I and its resolution still fresh - the world would be on the brink of World War II.