Had interventionist forces acted with greater decisiveness during the Second American Civil War, it is very possible that the Second Revolution would not have got off the ground. Had Prime Minister Antony Eden committed more British troops to stabilising New England in the early days of the collapse, it's possible that the region could have provided a suitable base of operations for the legitimate government to operate from.
Of course, Eden was dealing with the Second Indian Mutiny at the time, so it's somewhat implausible. Perhaps if 'Imperial Federation' had got moving earlier and preempted the violence in the subcontinent, freeing the British to commit fully to the New England Intervention? On an unrelated note, perhaps the Indian Empire would have remained an integral part of the Federation rather than simply consenting to retain the British King-Emperor as sovereign.
Continuing on that thought process, a more decisive Canadian contribution could also have been facilitated by an earlier Imperial Federation movement. Consider the implications of a Canadian government certain of support from the rest of what constitutes today's Federation (perhaps even including more African member states - an earlier shift in colonial policy could have convinced more to remain). IOTL, fears of antagonising a potential new regime to the south led to the rather lacklustre interventionist effort made. With an earlier Imperial Federation, a successful capture of Chicago in the name of the legitimate government could even be plausible, giving anti-communist forces an excellent foothold in the socialist industrial heartland.
Just my two-penny worth.