When was the latest a foreign power could have landed troops/invaded in the mainland United States?

I'm guessing sometime before 1920? The British, through their utter dominion of the seas + Canada? What about the German Empire prior to WWI?

And what would be the measure of success? If a British force landed and occupied D.C, NYC, etc. could that force the U.S. to surrender?
 
And what would be the measure of success? If a British force landed and occupied D.C, NYC, etc. could that force the U.S. to surrender?

Yes, the question requires clarification. It would be possible now to land a token force through merchant shipping (in the spirit of The Mouse that Roared), but it would last only hours until hemmed by the police.

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.
 
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).
 
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.

I agree. The plan of Imperial Germany to invade the US was delusional because (among other reasons) it assumed the US would simply sue for peace once a major city fell to a seaborne invasion.

To answer OP's question directly: I'd say that the UK, if it fully mobilized, could invade and substantially reduce the US during the Civil War. The economic and manpower parity would give the UK the advantage in such a war.
 
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