is there a justification for the us to engage in unilateral rather than multilateral military action overseas

Multilateral action is always better than unilateral, but that is not always possible. The United States, for instance, invaded Iraq with assistance from other countries. Unilateral action is taken if a country feels its national security interests must be protected at all costs. Even if one's actions are not supported by others, action must be taken.

Explanation: Because the world is becoming more global, it is necessary to align one's self with other nations in order to achieve certain goals.