World War One,Or The Great War, was fought by Generals who did not fully understand the mechanised killing machines that had been developed since the American Civil war, the crimean (both first use of trench warfare), the Zulu Wars of the late 1800's and the Boer war's in the early 1900's. Machine guns developed by Gatiling then perfected by Maxim, Lewis (who was an American dentist), Vickers and Spandau took a devastaing toll on troops ordered just to walk towards the enemy weighed down with kit. These later machine guns where light enough to be mounted onto the aircraft of the time. Also you had all sorts of gas,used on both sides, that either drowned you by filling up your lungs with fluid or blinding you. The tank was then being developed and in thier first outing poorly used on a muddy,shell pocked landscape while horses where still being used as the main reccon and line breaker unit. If you look at the treaties signed at the time Britain had a Entante Cordial it was not a treaty saying if you are attacked we will sent troops. So it was not a military pact just one based on comerse. While France had a military based pact with Russia. One of the main causes was that Germany,which had been unified by Von Bismarck, wanted to have a Empire based on the one that Britain had and a Navy similar the the Royal Navy. This lead in turn to both countries building bigger and heavier "Dreadnought" class Battleships so in a way that did contribute to a clash but not the main one. It was really the treaty between Russia/France and Germany/Austro Hungary that was the main factor of the start of WW1. If you look at the treaties signed at the time Britain had a Entante Cordial with France it was not a treaty saying if you are attacked we will sent troops. So it was not a military pact just one based on comerse. While France had a military based pact with Russia. While Russia had a close relationship,at the time, to the Balkan countries. So when the Archduke Franz Ferdinand got assainated all of mainland Europe mobilised. The Von Schleafen plan was based on taking France out then going for Russia, it did not take into account the British getting involved.
Actually you missed out the Spanish Civil War in your follow up question so there are 4. It was a testing ground for the recently formed Luftwaffe,secretly trained in Russia, with modern type fighters and bombers and a lot of Scots went to fight against Franco and Hitler. I think that the First World War did not really end until 1946 even though a lot of people do not know that the final peace was signed in 1919 not 1918. The peace paper had to be signed every month. The Versaille Treaty was a smouldering catalyst for the killing to begin again since the "Allies" wanted to punish Germany for what it did financally, "Black Jack" Perishing of the USMC Expidition Force had the right idea take the war to Germany just keep on going. That's what basicly happened in WWII even though envoys from Germany had been sent out with feelers for peace accords but the Nazi heirachy had thier heads so far up thier own backsides thinking that they where needed to keep the country in line and would not be brought to a war crimes trial. I reckon if you look at the whole it was part of the same thing so together it was just one great big tragedy and a waste of human lives.