A more thorough explanation:

Aggression is a serious violation of international law committed by a nation. It is considered a crime and is prohibited by international law. However, there is no universally accepted definition of aggression, despite many attempts to create one.

In 1974, the United Nations General Assembly adopted a resolution that defined aggression as the use of armed force by a state against the sovereignty, territorial integrity, or political independence of another country, or in a manner inconsistent with the Charter of the United Nations. This definition does not include measures that may constitute aggression in certain circumstances, nor does it recognize exceptional circumstances that would make the enumerated acts defensive rather than offensive.

Examples of aggression include direct aggression, which involves a state's regular armed forces, and indirect aggression, which is carried out by means other than through a state's regular armed forces. Indirect aggression can include delictual acts armed or unarmed and conducted vicariously by the aggressor state through third parties which endanger the essential rights of a state, rights upon which its security depends. It can also include delictual acts taken directly by the governing authorities of a state against another state or vicariously through third-party groups which do not involve the use of armed force, but which do endanger the essential rights of a state upon which its security depends.

For example, economic pressures on other states, demands couched in traditional diplomatic terms but laden with implied threats to compel action or inaction, fifth column activities, propaganda urging another state's peoples to rise against their government, aiding and abetting of rebel bands intent on overthrowing another government, and a wide range of other modern techniques can be included in the concept of aggression insofar as they are delicts at international law, for they are directed against the sovereign independence of a state.

Overall, aggression is a serious violation of international law that can take many forms, including direct and indirect aggression. It is important for nations to work together to prevent aggression and hold those who commit it accountable for their actions.