Final answer:
The long-term cause of the war involving the development of the armed forces and their use as a tool of diplomacy is militarism.
Step-by-step explanation:
Militarism refers to the glorification of the military and a readiness for war. In the early 20th century, European nations engaged in an arms race, building up weapons and military personnel. This increased the size and readiness of standing armies and naval forces, leading to tension and competition among the great powers.
For example, Germany sought to expand its colonial holdings and become a force in global affairs, which put it in conflict with other imperialistic powers like Great Britain. This competition for land and power led to military buildups and threatened the peace in Europe, ultimately contributing to the outbreak of the war.