I mean, the obvious thing that they can do is to make it practically impossible for the US to invade Canada. With c.50,000 regulars supported by c.100,000 Canadian militia, and with sufficient time to fortify key areas like Queenston Heights and Montreal, the British can force the US to look at raising an army of 300,000 - 400,000 just to have a chance of making gains in Canada. Under those circumstances, without the key strategic resources that Britain provides, and with the blockade likely to increase in severity, the probability is that the US cuts its losses and asks for peace. The whole point is to end the war by putting the US in a position where coming to the negotiating table is a no-brainer, with the added bonus that impressing the US with Britain's ability to defend Canada decreases the likelihood that Britain will ever have to actually fight to protect it.
Who's threatening Bombay or Cape Town in 1859? Is the US willing to risk New York, or Boston, or Portland, for possibly holding Toronto for six months until the peace deal? Is Georgia willing to have men killed to bring another few free states into the Union?
No it didn't.
Toss them in the bin, more like: peak Southern deployment in the summer of 1863 was less than 400,000 men (
1,
2,
3) This "
the US can field three million men" claim came up two weeks ago - do people now see what I mean about "if the people advocating the cause of the US were prepared to admit basic historical realities"?