Unilateralism
Unilateralism is any doctrine or agenda that supports one-sided action. Such action may be in disregard for other parties, or as an expression of a commitment toward a direction which other parties may find agreeable. Unilateralism is a neologism which is already in common use; it was coined to be an antonym for multilateralism, which is the doctrine which asserts the benefits of participation from as many parties as possible.
The two terms together can refer to differences in foreign policy approached to international problems. When agreement by multiple parties is absolutely required—for example, in the context of international trade policies—bilateral agreements (involving two participants at a time) are usually preferred by proponents of unilateralism.
Unilateralism may be preferred in those instances when it's assumed to be the most efficient, i.e., in issues that can be solved without cooperation. However, a government may also have a principal preference for unilateralism or multilateralism, and, for instance, strive to avoid policies that cannot be realized unilaterally or alternatively to champion multilateral solutions to problems that could well have been solved unilaterally.