Using borders and alliances as indicators of opportunity and willingness, respectively, we test the relationship between the sea and the diffusion of war during the 1816-1965 period. The impact of borders and alliances, individually and in combination, on the growth of ongoing war through infectious diffusion is shown through the comparison of baseline cases to cases where states at peace were exposed to various treatments comprised of warring border nations or warring alliance partners. The findings indicate that the probability of war diffusion is substantially increased as opportunities and willingness increase, particularly when such geographic and political factors are combined. The applicability of the opportunity and willingness framework to the study of war and diffusion is expanded and confirmed
