One historian wrote,"one long-range of schemes that appear almost accidental when viewed singly." Does this practice of imperialism in Africa and Asia substantiate this statement?

Yes

Explanation:

This statement alludes to the history of colonial expansion in African and Asian countries. This period was marked with the significant events in the 19th century where there was a need to expand the European territories in other countries beyond Europe. For example, China was divided into 20 spheres of influence. In the same way, Africa was divided into the same 20 spheres of influence with Britain taking the most from Africa.

Therefore, correctly spoken, the author and historian is correct in saying the statement.

To a point let me say that the practice of imperialism in Asia and Africa substantiates this statement because the Europeans did not know vividly what awaits them upon arrival, just that it was different than their everyday experience.

They couldn’t have fully expected what they would find which would make the success of their colonies an absolute accident, hence the practice of imperialism seems to substantiate the statement.

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.