This is the biggest question you could have asked about the First World War. The reason is because this is STILL under debate to this day. That being said here goes with some of the causes:
1) Imperialism – Britain still had an Empire at the beginning of WW1. Most of the other European nations had one too. Hey were very jealous of this and wanted a piece of the action. This caused tension between us and the other nations.
2) Complex relationships – At the beginning of WW1, all of the royalty of Europe was related to each other. Each country had different non aggression, mutual co-operation, and various other agreements. This made the situation a whole heap more difficult with how to navigate when conflict came to the service.
3) The Death of Arch Duke Franz Ferdinand – this is widely considered to the the catalyst for the start of the WW1. Without his death, the other nations would not have declared war (OR would have been at a different date)
Effects are:
1) Warfare changed forever – The first effects of the machine gun were seen. No more were there Calvary charges, and a whole new perspective of warfare was created. Military was no more about men running with guns, it was about artillery and more.
2) War Fatigue – People started to not want to go to war (In Britain at least). It changed out ideas on everything,. For the first time PTSD (Shell Shock) was brought to the front. Warfare was no longer looked at with the same rose tinted glasses.
3) Germany being ripped apart – The treat of Versailles was written with a lot of bitterness. I don't want to go into this to much as we already know the effect on Germany.
4) Drove technology forward – War always springs technology forward like nobodies business, and WW1 showed what technology could do on the battlefield.
