As is well known, marriage was frequently employed as an instrument of diplomatic policy in premodern Europe. Dynastic leaders used the marriages of their own family members to create or confirm alliances with other ruling houses. Peace was often the aim and the outcome of such agreements, but the reality of marital politics was far more complicated. Arranging a marriage could be a statement of enmity by two families toward a third party. Attempts to dissolve or prevent marriages already arranged by one's rivals amounted to viable political tactics. During the twelfth century, as rules around the formation and dissolution of Christian marriage were in flux, opportunities for manipulation of accepted practices abounded. This article examines a series of marriages between members of the Anglo-Norman and Angevin ruling families, focusing especially on the reign of Henry I (1100–35), to demonstrate the complexity of marital practices and their links to peace and war. Literary works of the age, notably Geoffrey of Monmouth's Historia Regum Britanniae, offer further cultural perspectives on the nature of political marriages. Whereas short-term tactical considerations meant that marriages were often tied to episodes of conflict and rivalry, from a longer-term perspective the usefulness of marriage as a means of securing and maintaining peace remained an important element of premodern diplomacy.