What is aggression?
The UN definition is based on actions not words - a declaration of war doesn't feature in the list. Instead it includes such acts as:
the invasion or attack by the armed forces of a State of the territory of another State
any military occupation, however temporary, resulting from such an invasion or attack
any annexation by the use of force of the territory of another State or part thereof
bombardment by the armed forces of a State against the territory of another State
the use of any weapons by a State against the territory of another State
blockade of the ports or coasts of a State by the armed forces of another State
attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State
use of armed forces of one State which are within the territory of another State with the agreement of that State, in contravention of that agreement (e.g. overstaying permission for the army of state A to stay on the territory of state B)
the action of a State in allowing its territory to be used by another State for perpetrating an act of aggression against a third State
sending by or on behalf of a State armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such seriousness as to amount to the acts listed above