The article examines the mechanism of action and implementation of the principle of international cooperation in modern international relations against the background of the European integration of Ukraine and the strengthening of the sphere of security and defense. The content and legal and organizational foundations of the principle of international cooperation are analyzed. Trends in the implementation of the principle of international cooperation against the background of the European integration of Ukraine and the strengthening of the sphere of security and defense have been studied. The opinion is argued that international organizations, due to their nature and external factors, do not always act as effective moderators of international cooperation. Aggression against Ukraine exposed a number of fundamental problems related, in particular, to the responsible attitude to the observance of the principles of international law and the consequences for their violators. The essence of the principle of international cooperation has changed, when preference is given not to universal elements of the system of collective security, but to direct contacts with the member states of the UN, EU, NATO, and other alliances, which allows to effectively solve urgent issues in the field of security and defense. It is proven that one of the trends in the implementation of the principle of international cooperation is the strengthening of the institution of responsibility and the binding factor against the background of efforts to establish the right of the strong in international practice by some member states of the UN. Aggressive actions of Russia, the discrediting of the norms of international law, the commission of international crimes on the territory of Ukraine and other factors encourage us to once again turn to the problem of the introduction of binding (sanction) components in interstate relations. On the agenda of the international community are issues of reorganization of the system of collective security, strengthening of the institution of responsibility, creation of effective mechanisms for limiting countries that violate the principles of international law. Without the application of the principle of international cooperation, including at the regional and local level, it will be extremely problematic to solve these extremely complex issues.