In the late medieval period, as the nation became unsettled, kosen bosen (offensive and defensive war) broke out frequently to resolve disputes over the boundaries of territories or the like by force (self-help) instead of by bringing them into court.
中世後半になると、社会が不安定となり、境相論などを訴訟によらず実力によって解決（自力救済）しようとする故戦防戦が頻発するようになる。