This article’s example of committing an act of war against an autonomous and self-determined nation (with which they are not formally at war with) is a good start.

If you’d like more: The forcible relocation of indigenous civilians in contested or unowned territory. The kidnapping and murder of individuals deemed as war criminals from uninvolved third party states not at war with either side. Subversion of governments deemed unfriendly in the Middle East. Feeding false intelligence to allied nations including the US (see WMDs) to instigate conflict favorable to their expansion. Providing weapons, training, and funding to non-state actors to destabilize competitors.