I believe this is best answered on page 152, where Loewen cites;

“For our first seventy years as a nation, then, slavery made our foreign policy more sympathetic with imperialism than with self-determination. Textbooks cannot show the influence of slavery on our foreign policy if they are unwilling to talk about ideas like racism that might make whites look bad. When textbook authors turn their attention to domestic policy, racism remains similarly invisible.”

You might want to go over this entire section, as the above question is a bit complicated for this forum.