"The land acquisitions and dislocations of Native Americans and other residents that were a product of Manifest Destiny were critical to the national interests and securtiy of the United States and therefore justifiable." To what extent was this expansion justified?
The national interest of a country is a pretty subjective term. That is, it means different things to different people. The United States at the time of westward expansion was facing no serious threats from outside the country. The driving forces behind Manifest Destiny were economic and religious in nature. Motivated by greed, justified by religion.
What's more, expanding into these new territories set off a bloody national debate and increased sectional tension over the issue of slavery, ultimately leading to secession and the Civil War, none of which was positive for the interests and national security of the country.
At the time when this expansion was happening, people would have agreed with this statement. Today, however, we would generally say that this statement does not reflect acceptable attitudes.
It is certainly true that the territorial expansion was in the US's economic and security interests. It gave us very defendable borders with no major important hostile powers on our borders. It gave us land that was hugely important to our economy (California Gold Rush, Comstock Lode, oil in Texas, etc). In the 1800s, this was seen as a good reason to take the land given that we thought of the Native Americans and Mexicans as backwards.
Today, however, this is not an acceptable attitude. We no longer believe that taking other people's lands just because it is good for us is justifiable. Therefore, this expansion would be unjustifiable if it happened today.