Arcturus Mengsk I, Emperor of the Terran Dominion
Arcturus Mengsk was originally a prospector for the Terran Confederacy. Hailing from the outlying world of Korhal, Mengsk was the descendant of many anti-Confederate political activists. At the height of the anti-Confederate movement there, the Confederacy launched a huge salvo of Apocalypse-class nuclear warheads at Korhal, decimating the planets surface and killing Mensgk's entire family. Vowing revenge, Mengsk resigned his government commission and formed the insurgent group the Sons of Korhal.
With the help of Sarah Kerrigan, former Confederate Marshal James Raynor, and a former Confederate Magistrate, Mengsk was able to press into service General Edmund Duke of Alpha Squadron and overthrow the Confederacy. Using Transplanar Psionic Waveform Emitters, Arcturus lured the Zerg Swarm to the Confederate capital of Tarsonis, which was overrun and then incinerated by the Protoss Expeditionary Force, under the command of then-Executor Tassadar. Incidentally, Mengsk's negligence led to Kerrigan's capture by the Swarm and her rebirth as the Queen of Blades. After this occurred, James Raynor, now Captain of the Hyperion, left Mengsk's service and joined Tassadar and the rest of the Expeditionary Force.
Mengsk was able to use the military might of Alpha Squadron and the Sons of Korhal to establish a new government on Korhal after the fall of the Confederacy. Calling it the Terran Dominion, Mengsk named himself Emperor of the Koprulu Sector. (His coronation speech is transcribed below.) Shortly after the establishment of his Dominion and the later fall of the Overmind on Aiur, the Koprulu Sector was invaded by the fleet of the United Earth Directorate, which laid waste to the capital of Augustgrad and nearly captured Mengsk himself.
At the direction of Kerrigan, now the Queen of the Zerg Swarm, Mengsk was rescued by James Raynor and his friend Fenix, a formed Protoss Praetor. Mengsk assisted Kerrigan in destroying the UED Psi Disruptor, and in return she helped him reclaim Korhal from the UED. She betrayed him, however, and killed General Edmund Duke on Korhal. This led to Arcturus assembling a large fleet and attacking Kerrigan's base planet of Char. He, along with the UED and Protoss forces, was repelled by the Zerg Swarm. He currently remains on Korhal, rebuilding the Dominion.
Coronation Speech of Arcturus I, Emperor of the Terran Dominion
Transcribed from UNN Broadcast
Fellow Terrans, I come to you in the wake of recent events to issue a call to reason. Let no human deny the perils of our time. While we battle one another, divided by the petty strife of our common history, the tide of a greater conflict is turning against us, threatening to destroy all that we have accomplished. It is time for us as nations and as individuals to set aside our long-standing feuds and unite. The tides of an unwinnable war are upon us, and we must seek refuge on higher ground, lest we be swept away by the flood.
The Confederacy is no more. Whatever semblance of unity and protection it once provided is a phantom; a memory. With our enemies left unchecked, who will you turn to for protection? The devastation wrought by the alien invaders is self-evident. We have seen our homes and villages destroyed by the calculated blows of the Protoss. We have seen first-hand our friends and loved ones consumed by the nightmarish Zerg. Unprecedented and unimaginable though they may be, these are the signs of our time.
The time has come my fellow Terrans, to rally to a new banner. In unity lies strength; already many of the dissident factions have joined us. Out of the many, we shall forge an indivisible whole, capitulating only to a single throne. And from that throne, I shall watch over you!
From this day forward, let no human make war on any other human. Let no Terran agency conspire against this new beginning. And let no man consort with alien powers. And to all the enemies of humanity, seek not to bar our way, for we shall win through, no matter the cost!