What is required in order to create a world war

In order for a world war to begin, a number of things must happen. So all the countries would be in a postion that would cause a war, countries must have some sort of alliances in place that would make them obligated to help fight. Alliances could come from defensive pacts, trade aggreements, or old hatreds started from old wars. There must be at least two large groups of countries that don't nessarily like one another. With the allainces inplace several economic, culture , and possibly military factors must cause tenstion between the groups. With tenstion and obligation to allies, a incident must insue which begins the war. Incidents include: assasanation, invasion, accidents, terrosist attacks, bad trade deals, and other events which offend alliances. This all could happend today,but with the Un implaced probally wont.