The heir to the throne of Austria-Hungary is assassinated, and eventually the whole world becomes entangled in a conflict that few even wanted.

Wikipedia has a useful time line of the events and how the alliances pulled one country in and another then another until Russia, Britain, France and eventually America were fighting Germany, Austria-Hungary and the Ottomans.

It was a time period that bridges the old world with the new. Calvary charges against machine guns. Men charging in ordered lines and then mowed down and killed.

Defensive positions had a decisive advantage. But it took the generals time to realize what did and didn’t work with the new technologies. The time cost millions of lives and created a living hell for the soldiers. The number of losses were multiples more then previous wars.

The Germans decided on open submarine warfare, which was disastrous for Britain until they discovered the idea of traveling in convoys.

Russian experienced it’s revolution and the old government collapsed resulting in a treaty freeing up the German eastern front. The Germans decided on a last ditch offense, but the Allies were able to drive back the Germans forcing a surrender.

The pointless battles and the pointless loss of human life. It was such a huge waste and in retrospect it accomplished little besides setting up the stage for world war 2.