Was the Monroe Doctrine a justifiable policy for the United States?
In theory, the Monroe Doctrine was much more justifiable than in practice. In addition, it was a much more justifiable policy in the context of the 19th century than it seems today.
On its face, the Monroe Doctrine is eminently justifiable. All that it says is that European countries should not come in and try to colonize the newly independent countries of Latin America. It does not say that the United States should be able to dominate those countries or to interfere in their affairs. This is completely justifiable. However, the doctrine has been used to justify US interference in the affairs of other countries. It has even been used to justify occupations of those countries. This is much less justifiable.
We must also note, however, that such domination of other countries was far from rare in the 1800s. While such actions would be met with great disapproval today, they were commonplace in their time. Therefore, we should be at least somewhat careful about saying the policy was unjustifiable since that involves imposing our values and attitudes from today on people from the past.