In 1863, the British Empire took advantage of the distraction created by the American Civil War and supported the French in its invasion of Mexico in support of Maximilian I. With the earlier combined effort of the British, Spanish, and French forces, Mexico fell to French control by 1865. Full control was established within a year. The British had lessened their involvement in Mexico after seeing not much return. The likelihood of the American Civil War becoming a long drawn out affair became more assured and they sensed profit. The British Empire sought to invest in the war for the future. Considering the CSA’s early success against the Union, Queen Victoria decided to side with them with particular conditions. Slavery was a major issue for them. After some intense negotiations, a treaty was struck between the two. Jefferson Davis agreed that the CSA would slowly phase out slavery but it seems that agreement was made without any intention of following through. It seems he thought that the economic benefits of the alliance would outweigh the human rights issues. They begin to send troops and naval support to the South in 1867. They participate in a few battles but their assistance is too late for the CSA.
The Battle of New York was the turning point. The Union had managed to create the first crude Gravicite weapons technology which was enough to decimate the British forces. The UK was unable to provide enough support to prevent the CSA from falling to the Union. As the CSA descends deeper into ruin, they refuse to honor their treaty with the UK. Frustrated by the wasted opportunities over the last few years, the UK throw their effort into a last ditch attempt to grab some territory. In a lightning raid, the northern part of Maine is taken. They declare the territory the Commonwealth of New England. The Union gears for another conflict but Andrew Johnson caves to pressure from several sources and acquiesces the territory to the British Empire. English subjects begin traveling to America and a mass exodus of Americans begins. This sudden population shift instigated a massive effort to spread West and develop the East coast cities to accommodate the change.
At first, the Commonwealth was ignored by the Union and CSA. There was a tense border situation in the North but it never boiled over. After the Union and CSA merge into the United States of America, the Commonwealth gets recognized and trade opens. As the trade relationship builds, the British Empire receives permission to build a facility near NYC in order to facilitate better trading. Within months, an agreement is reached between the US and the Empire to make a part of NYC joint territory to solidify the economic relationship between the two powers. By 1889, they have become solid allies but the future looks shaky as they both seem to poised to get in each others way in their march towards the future.