What led to the disastrous stalemate of the trench wars between the great powers (WWI)?

This was the first "modern" war in terms of weapons. It brought about the machine gun, tank, chemical agents and airplane and lost the horse. In essence, neither side knew what to do with these machines, so they lined up on two sides and waited for the other to move. This caused mive casualties as men were just blown apart if they ventured into no man's land between the two sides. Basically, to gain ground, commanders had their men run as far as possible, with a few making it to the next trench alive. It wasn't pretty.