Palmerston's War: 1862

As noted previously, we've been asked to contribute to an alt-history short story anthology. We've decided to do a Palmerston's War short story.

The idea is that the Trent Affair, in which the USN boarded a British ship and removed two Confederate diplomats, leads to war. This was a real possibility. Cooler heads prevailed, though.

In our estimation such a war would have been about vulnerabilities. Of course the United States could have walked into Canada anytime it wanted. Conversely, the East Coast was wide open to British naval action. 

So the matter comes down to who can do what first.