The main cause of World War 1, the cause that caused all other causes was the Industrial Revolution. The Industrial Revolution caused the United Kingdom to realize that they needed more resources and through imperialism could get them. They could use colonized countries for free market and exploit them for resources. Imperialism, once Germany was developed and wanted to colonize, their options were limited, this caused tension and economic competition. Now once one country declared a war, another country would join in because they were allies. This all led to WWI.
The advancements that were made during the Industrial Revolution helped along Militarism. New weapons were being built and everyone had access to them. This made the war even more aggressive and it cause more deaths.