The Iconian Resistance was not exactly an alliance based on territory, but a resistance movement composed of several interstellar governments, formed to oppose the returning Iconians and their servitor races who sought to conquer the galaxy, leading to what became known as the Iconian War of 2410. It consisted of forces from the Alpha, Beta and Delta Quadrants, and ultimately involved the Gamma Quadrant as well. (STO video game: The Iconian War)
The origins of what would become the Iconian Resistance group dates to the early days of 2410, when M'Tara appeared at the Great Hall on Qo'nos after the Undine conflict with the Federation-Klingon-Romulan alliance ceased, wich made the newly formed alliance realize that the Undine attack was actually an Iconian plot to weaken the other species before their return. When M'Tara left Qo'nos, the Federation and the Klingon Empire decided to cease fire after five years of conflict, realizing their fight would ultimately destroy both sides. Together, the United Federation of Planets, the Klingon Empire and the Romulan Republic officially formed the Alpha Quadrant Alliance in order to rise against the Iconian Empire threat to the galaxy and any other which would stay in the Iconian's side.
After the beginning of operation Delta Rising, the mistery of an unknown foe known to be creating caos through the entire quadrant revealled to be Vaadwaur Supremacy's work. The Vaadwaur people had actually been helped by the Iconian Empire, wich gave enough technology for them to start a campaign of conquest through the region. In order to end the Vaadwaur threat, the Alpha Quadrant Alliance organized a meeting with several species from all over the Delta Quadrant. This utimately led to the formation of the Delta Alliance, with the main goal of defeating the Vaadwaur Supremacy once and for all.
Sometime after the Iconian Empire's invasion began when a Herald fleet was sent to attack New Romulus, an alliance lead on the Krenim Imperium - wich had been erradicated by the Supremacy - ultimately led them to Kyana Prime, where allied officers discovered a hidden Krenim colony hidden out of normal timestream, the Krenim agree to work together with the alliance in order to recreate a temporal weapon originally designed by Krenim scientist Annorax. This led to the Krenim Imperium's alliance, the Krenim Coalition, to combine forces in order to defeat the Demons.
During the decisive battle to who would win the Iconian War, the United Federation of Planets sent all available ships to defend its capital planet, Earth. Forces from all three quadrants of the galaxy were present to aid in the battle. During the confront, Sela returned from her quest to find allies in the Gamma Quadrant, bringing with her an entire fleet of Dominion ships, wich helped the Resistance on defending the Krenim Ship Annorax while it created a time portal to ancient Iconia. After returning from the past with the World Heart in hands, the Iconian Empire agreed to cease fire and retreat to Iconia, where they would begin their work on rebuilding their once peaceful civilization.