aggressor

[ uh-gres-er ]

/ əˈgrɛs ər /

a person, group, or nation that attacks first or initiates hostilities; an assailant or invader.

Origin of aggressor

1670–80; < Late Latin, Latin aggred- (stem of aggredī to attack; see aggress) + -tor -tor

And in many cases the appropriate response to an invasion has been a declaration of war against the aggressor.

Is the U.S. Enabling Putin's Invasion? |Christopher Dickey |August 29, 2014 |DAILY BEAST
When passions are enflamed, people will be less willing to sit down and negotiate with someone they view as an aggressor.

Venezuela Burns, Body Count Rises |Juan Nagel |March 16, 2014 |DAILY BEAST
Davis begins the film, punched by an aggressor into the gutter and ends it the same way.

Why No Oscar Love For 'Inside Llewyn Davis'? |Tim Teeman |January 20, 2014 |DAILY BEAST
Gladwell claims that the officer was trying to restrain his dogs while Gadsden was the aggressor.

Malcolm Gladwell In Five Minutes: What to Know to Pretend You’ve Read the New Book |Thomas Flynn |October 5, 2013 |DAILY BEAST

Going back to square one would be an unjust outcome and a prize to the aggressor.

Excitement Over Land Swaps Is Misplaced |Dani Dayan |May 7, 2013 |DAILY BEAST
Plato approves of war conducted so far, as to compel the aggressor to indemnify the injured and the innocent.

The Rights of War and Peace |Hugo Grotius
Yet the wounded man, getting hold of his cattan, wounded the aggressor.

A General History and Collection of Voyages and Travels, Volume IX. |Robert Kerr
It is in this way that nature seems to prepare a punishment for the aggressor.

Economic Sophisms |Frederic Bastiat
Thus the majority of the Chamber, with the clearest warning of danger, insisted on giving the aggressor every possible advantage.

History of Modern Europe 1972-1878 |C. A. Fyffe
From the start, however, Sheridan was the aggressor and Hampton was forced to fight a defensive battle.

Personal Recollections of a Cavalryman |J. H. (James Harvey) Kidd