Why can arms races increase fear and hostility between rival countries?

Arms races increase fear and hostility between rival countries primarily through the security dilemma, a concept in political science where actions taken by one state to enhance its own security inadvertently threaten others.1

Because there is no world government to regulate international conduct, states must prioritize their own security. When one country procures weapons or develops new military technologies, rival states cannot be certain of the arming nation's future intentions. Consequently, these rivals often respond in two ways that escalate tensions:1

• Competitive Buildup: Rivals increase their own military capabilities to reestablish a balance of power, leading to a "security spiral" where both sides arm themselves increasingly heavily.1
• Preemptive Action: Fearing that the arming state might eventually upset the balance of power, rivals may consider launching a preemptive attack to neutralize the threat before it grows.1

This cycle of reciprocal military expansion often reflects an adversarial relationship, turning defensive measures into sources of mutual suspicion and long-term instability.21