Nationalism had Europeans each viewing their own nations as better than the others, in competition with the others. This would lead to an increase in tension between the nations.
Imperialism carried that nationalistic rivalry to other parts of the globe. Nations sought to grab control over parts of Asia and Africa. When war erupted, that also made it a world war, because people from imperial territories would be brought into the war.
Militarism included a massive arms race -- expanding armies and navies. The construction of so much more military hardware made the coming of war more likely.
Alliances were being arranged, often through secret negotiations, with terms of the alliances known only to those who were involved in the negotiations. Sometimes the fact that alliances were being made was announced publicly. This business of conspiring in regard to alliances caused escalating nervousness and tension in Europe. This was a fundamental problem that lay behind the ultimate outbreak of the Great War (as World War I was called at the time it occurred).
