I'd be interested in how people think that might actually have played out.
If we assume that the British are intervening in any significant way, that is that they are trying to actually win the war for the Confederates (without losing Canada in the process) I think the following assumptions can be made:
- Much of the available British Army will be deployed to Canada, with Canadian troops in support.
- Assuming the planned first Canadian militia callout is fully subscribed to, as expected, and that the second callout is not fully subscribed to, then combined with volunteers there will be upwards of 60,000 troops generated from the Province of Canada and another 12,000-15,000 from the Maritimes.
- This is in addition to a likely minimum of 50,000 British troops to Canada.
- The amount of force this represents will prevent the US from taking Canada easily. It will mean the US has to fully commit to an attempt to take Canada, and thus strip significant troops from the fighting front against the Confederacy to do so; even if the US does not commit to this they will have to send several divisions to the Canadian border.
- There will be a blockade. It will be sizeable. It will cut off much of US foreign trade (imports and exports) and much of the domestic coasting trade.
- US imports of rifles, gun iron, steel, railroad iron, saltpetre, lead, sulphur and other war-fighting materiel will suffer. In some cases it will be completely cut off and the US will have to make do with existing stocks and sometimes-inadequate local production.
- US exports of grain will suffer. If the US continues to sell grain to third-party shipping this will probably be allowed to continue and the disruption relatively small, but if the US passes an Embargo Act then the export market will collapse and there will be a lot of unhappy farmers.
- The Royal Navy will concentrate multiple ironclads and gunboats to attack US ports, in order to disrupt and destroy US building capacity.
- This will also serve to act as a third "pull" factor for US warfighting capacity, away from the fighting front against Canada and away from the fighting front against the Confederacy.
- To aid this the British will concentrate a sizeable sea-mobile force (in at least division strength, i.e. 2x 3-battalion brigades) which can support these raids and provide limited inland striking capability.
- If the US does not deploy sufficient troops in any of those potential coastal targets to prevent greater British offensive action, then the British will exploit this lack.
- If the US does not deploy sufficient troops along the Canadian border to prevent British offensive action then this lack will also be exploited.
- The Confederacy will recieve a sizeable upgrade in terms of weapons (small arms and artillery), ammunition (saltpetre) and possibly warships as well.
- They may also recieve the benefit of British musketry instruction, and even the deployment of a British corps (12 battalions in two binary divisons), probably to the eastern theatre.
- The British are likely to deploy a small expeditionary force out of Indian Establishment against San Francisco, and to blockade the port.
- This force is likely to include both British and Indian battalions.
- If the war continues or seems likely to continue for a significant period of time, the British will raise new battalions of troops.
- These will mostly consist of regularized militia infantry and Volunteers (i.e. already partly trained capable second-line home defence units) which will then be further trained to a higher level before being deployed.
- The British will also likely build new ships specifically for the war.
- These will consist of:
-- gunboats and gunvessels upon the Crimean pattern, mass produced ships with a few medium to heavy guns.
-- ironclads upon the Crimean pattern, quick to build and heavily armoured with heavy guns but low engine power.
-- Mortar vessels for coastal attack.
It is extremely difficult for me to see how the US could do damage sufficient to knock the British out of the war. The minimum required is probably an invasion of Canada, but there is no point in the Civil War when the US has sufficient capacity lying around uncommitted to allow for an invasion of a defended Canada while also defending the coastline. The US will
have to make compromises.