Final answer:
Military alliances, nationalism, and imperialism were the three forces that set the stage for war in Europe, leading to heightened tensions and the eventual outbreak of World War I.
Step-by-step explanation:
The three forces at work in Europe that helped set the stage for war were military alliances, nationalism, and imperialism. These factors contributed to tensions and competitions among European powers in the early 20th century. The intricate system of alliances, such as the Triple Alliance of Germany, Austria-Hungary, and Italy, and the opposing Triple Entente, bound countries together and raised the potential for a regional conflict to escalate into a widespread war. Nationalism, the drive for nations to assert their sovereignty and interests, fueled rivalries and territorial disputes. Imperialism led to competition over colonies and global dominance, particularly evident in Germany's ambitions to expand its colonial empire, which clashed with established powers like Great Britain. Together, these forces set Europe on a path towards the catastrophic conflicts of World War I and beyond.