To be fair the US Merchant Marine on the eve of the Civil War was fairly respectable and carried perhaps two thirds of America's overseas trade. I am trying to find a good data source. What I do know off the top of my head is that the US fleet was still largely sail driven and had been relatively slow to adopt steam possibly because of a tradition of building arguably the best commercial sailing ships in the world?
The big issue is of course a lack of global infrastructure necessary to support sustained deployments.
Well in the
1850s the US had 1.5 million tons of merchant shipping engaged in foreign trade, then 2.5 million in 1860, but due to the Civil War it dropped by 1 million tons (and was actually about 60,000 tons less than two decades earlier) by 1870. Perhaps not coincidentally Norway's grew by 500,000 tons in the same period.
You're right however, the limiting issue is the independent overseas infrastructure which would allow them to conduct independent operations in war time. Reaching forward to 1905, when the Russians sent portions of their Baltic and Black Sea squadrons to replenish the destroyed Pacific Squadron, they were denied use of the Suez Canal and British bases, which notably impeded their performance and efficiency when they did meet the Japanese at Tsushima. The US, lacking similar infrastructure in this period, would be hard pressed to make its presence felt around the world without access to foreign ports to coal and resupply.
a lot of this discussion is getting off the mark... which is, 'can the US enforce the MD during the ACW"... I think we've answered it pretty well as 'no, not against the UK or France'. And those two nations are the biggest deep sea powers at the time. Any lesser naval power who has a mind to violate the MD is going to have to face the UK; while the Brits weren't willing to make France back down for assorted reasons, they are likely to make smaller powers back down... so, the US won't need to do it on their own...
Good points. Spain was also the exception since she had pre-existing colonies in the Caribbean. Though that does raise the question of would powers like the Netherlands and Denmark have been excluded as they all had Caribbean territories too.