Large and rapid power shifts resulting from exogenous economic growth are considered sufficient to cause preventive wars. Yet most large and rapid shifts result from endogenous military investments. We show that when the investment decision is perfectly transparent, peace prevails. Large and rapid power shifts are deterred through the threat of a preventive war. When investments remain undetected, however, states may be tempted to introduce power shifts as a fait accompli. Knowing this, their adversaries may strike preventively even without conclusive evidence of militarization. In fact, the more effective preventive wars are, the more likely they will be launched against states that are not militarizing. Our argument emphasizes the role of imperfect information as a cause of war. It also explains why powerful states may attack weaker targets even with ambiguous evidence of their militarization. We illustrate our theory through an account of the 2003 US-led invasion of Iraq.