1) Belligerence 2) Bombing 3) Invasion 4) Pearl Harbor attack 
Any act occurring in the course of declared war; armed conflict, whether or not war has been declared, between two or more nations; or armed conflict between military forces of any origin. 18 U.S.C.
In business, an act of war is anything that causes loss or damage as a result of hostilities or conflict. Such risks are excluded from all insurance policies (except life assurances). In marine and aviation insurance only, any extra premium may be paid to include war risks.
an act of aggression by a country against another with which it is nominally at peace.