"Windows of opportunity" has been a core concept in the security studies literature for a long time. It generally refers to a period during which a state has significant advantage over a rival. In turn, this advantage is thought to encourage aggressive behavior by states looking to subdue their opponent. Despite its prominence in scholarly and policy writings, few studies have actually tested the "windows" argument in a systematic way. Rather, it is treated as an article of faith among many scholars that shifts in military capabilities generate windows and these windows trigger aggressive behavior. Military factors, however, are only one—and perhaps not even the most important—component of a window of opportunity. Recent work on the role of third-parties in international disputes suggests that diplomatic considerations play an important part in decisions of war. A state would be reluctant to risk war against a rival if doing so might prompt the intervention of third-parties who could influence the war's outcome. Conversely, leaders who anticipate outside support or indifference are more likely to take such risks. Shifts in diplomatic relations, then, may also open and close windows of opportunity. Absent from the literature is any effort to disaggregate and compare these different types of windows—military and diplomatic. Yet, this distinction is an important one. It points in two different and often opposing directions for understanding and predicting aggression. I attempt to fill this gap.