Coalition historians would say that the Romulans started the war with an unprovoked attack upon a number of Earth colonies and deep space station. Romulan historians would say that the attack and following war was necessary to protect themselves from Earth and their military formed alliance.
Both versions are true in their own way.
The Coalition was formed in responce to increased aggression from the Romulan Star Empire. After the disruption of the Babel Conference in 2155 the Confederacy of Vulcan, Andorian Empire, Tellarite Republic and United Earth started to work together as equals and friends forming the Interstellar Coalition in the efforts of mutual peace, protection and exchange of ideals and knowledge. In the span of a few short years, the Coalition had become a major force in the beta quadrant.
However after the Romulan's failed attempt of re-unification with their Vulcan siblings thanks to Earth, they turned their attention to the Humans. With Earth expanding towards their territory with colonies and bases being developed at an alarming pace, the Senate and Praetor needed to act in preseveration of the Romulan people and their allies. In the span of 12 hours on March 15th 2157, the Romulan Star Empire attacked the Earth colonies of New Elysium, Alpha Centauri, Vega and Deneva and destroyed Salem One Station, Deep Space Two and Cold Station Five. Included in the casualities was the destruction of the UES Enterprise, Earth's first warp five starship. With this swift attack, the Romulan people showed the Humans that they will not tolerate any interfearence or expansion towards their territory.
With the reports of the attacks, the United Earth President Anastasia Molyneux, with a passing resolution from the United Earth Congress, declared war aginst the Romulan Star Empire.
"Four years ago, Earth was viciously attacked without provocation by an alien species manipulated into thinking that humanity was their enemy. That date, March 24th, joined December 7th, September 11th, July 7th, and October 1st as a date that will forever be seared into our consciousness. Now, another date has joined them, and it too will live on in infamy.
"Yesterday, March 15th, 2157, four human colonies and three human space stations were suddenly and deliberately attacked by the forces of the Romulan Empire.
"We were not at war with this species, nor was Earth infringing upon their sovereign territory. At no time have we harboured any ill will toward these our brothers from the stars, and have even extended the hand of friendship to them in every way we can. And yet, despite our attempts, we have been rebuffed in the most savage way imaginable.
"It must be noted that the distance between the seven locations that were attacked, and the timing of these attacks makes it obvious that this was deliberately planned and cannot be construed as anything but an act of war. We do not have an accurate death toll yet, but early estimates place it in the hundreds of thousands, with as many as one and a half million possibly slain in an unprovoked attack.
"Therefore, as the commander in chief of United Earth forces, I have directed both Starfleet and the Military Assault Command Operations to take any and all measures for our defence. I have further asked that the Congress declare that a state of war now exist between the territories of Earth and the Romulan Empire.
"At 1700 Hours Zulu today, Congress has responded and passed a unanimous resolution that backs this request.
"As of this moment, Earth is at war.
"And may God have mercy on our souls."
-Excerpts from UE President Anastasia Molyneux's speech before Congress,
March 16, 2157.
With Earth leading the way, and the rest of the Coalition joining Earth as per terms of the Coalition Charter, Earth found itself in it's first interstellar war.
