I had this idea for a timeline for me.
The US annexes Mexico. Central America up to the border of what is now Panama is US protectorates, as well as Cuba and Puerto Rico (ignore -how- it's done, but I'll say the autonomist movements in Cuba is as strong as those in Puerto Rico, a bit more & Mexico was annexed by the 'indios' being granted reservation and linguistic autonomy in mexican highlands and Maya regions + Mestizos being given lands in the US 'plains states'.
how the war is set up by me:
POD I: Incident between British and US during US civil war results in a shooting war between Britain and the US which is contained.
POD 2: for war: The incident between the British and the US over Venezuela's claim on British Guiana. The US backs up Venezuela against Britain. Somehow, this (uber-wanked?) US conquers Canada, Jamaica, the Bahamas from the UK and possibly some british islands in the Carribean (US-British war replaces US/Spanish War ITTL).
The Venezuelans, with US back-up conquer British Guiana. The Brazilians also use this to annex French Guiana, Venezuela and Brazil partition/divide Guianas between themselves; and the US 'shields' them/aids them from French and British counter-attacks.
EDIT: this is related something I found
http://real-life-alt-history.blogspot.com/2011/03/french-guiana-belonging-to-brazil.html