Before WWI, the United States Army was pitifully small compared to the European Great Powers. Even Belgium had a superior army. Invading the United States would have been easy, at least initially.
However, I don't think it's realistic to believe that the seizure of a coastal city would lead the US to seek peace. The country would probably mobilize en masse from sea to shining sea to drive out the invader. The invading army, once it realizes that the US will not surrender, will probably abandon whatever city it has occupied before it is annihilated.
How about this as a measure of success: hold a major US city for six months. I'm going to guess about the 1880s. If the Royal Navy can take San Francisco by surprise, it would still take time for the US to deliver troops by rail in sufficient numbers.
The West Coast would probably be the most difficult for the US to defend. However, its location would mean that an invasion could be detected from thousands of miles away (literally).