How significant was the Monroe Doctrine when it was announced?
When it was announced the Monroe Doctrine had very little significance at all. The US completely lacked the power to enforce the doctrine and could not seriously have hoped to prevent any major European power from colonizing in South America.
The only reason that the Monroe Doctrine seemed to have any weight was that it coincided more or less exactly with the interests of Great Britain. Great Britain, with its powerful navy, could enforce the doctrine. It would be at least 70 years before the US had sufficient power to do anything to enforce the doctrine.
The only real significance of the Monroe Doctrine (it wasn't even called by that name until the 1850s) was that it showed a rapprochement between the US and Great Britain. The Monroe Doctrine showed the US siding with Great Britain in terms of geopolitics.
So, the Monroe Doctrine had little international significance because the US was in no way a major power at the time.