WW1 was thought to be a quick battle with hardly any resistance offered to the Germans. But something went wrong, very wrong for the fight that was thought to be over in 43 days on the western front ended up as a shoot out for four long years. There were many reasons for this, mainly the failure of the Schlieffen Plan Also new technology and barley any supplies on the front line. All these things lead to the stalemate that was formed and the main reason the war went on for so long.
This was the first time the whole world got pulled into one fight. Before there had only been small wars broken out and solved by the countries/states at war. Now, many countries were pulled in to the war without any aims or goals for the war. They only fought because they had signed a treaty saying they would fight if their allied was attacked. This meant that many of the countries entering the war were not prepared for what would happen. One good example of this were the French, who started the war wearing bright red jackets.
But this can not be blamed for keeping the war going for so long.I think the main reason for the stalemate was the Germans being unable for use the Schlieffen plan correctly. The Schlieffen plan meant that the Germans would attack Paris from the east, going through Belgium and all the way out to the Channel to prevent British troops entering the war. The Germans planed to capture Paris within 43 days. They didn't get closer than 40 miles during the whole four years. The main reason for the Schlieffen plan failing was the unexpected Belgian resistance. The Germans took 19 days to capture Brussels and by that time the French had mobilised and organised themselves. The French managed to keep the Germans away from the important Channel ports which made it possible for the British to enter. The British Entered the war due to a treaty they had signed with the the neutral Belgium, saying they would defend them if they got attacked