Final answer:
The Monroe Doctrine was ineffective initially because the United States did not have the military strength to enforce it against European countries.
Step-by-step explanation:
The Monroe Doctrine was relatively ineffective for much of the 19th century mainly due to the United States' lack of military power to enforce it against European nations, had they chosen to challenge its declarations. Originating from President James Monroe's annual message to Congress in 1823 and drafted by Secretary of State John Quincy Adams, the doctrine asserted that future European colonization in the Western Hemisphere was unwelcome, and that the political systems of the Americas and Europe should remain separate.
Although it was a statement of autonomy and non-intervention, at the time, the United States did not possess the naval or military capacity to support such a doctrine against any confrontational European power.