What is the act of canceling out a federal law?

What was one major cause of worldwide conflict during the first decade of the 21st century?

Why were presidents andrew jackson and martin van buren unwilling to annex texas? a. they did not want texas to be admitted to the union as a slave state. b. they feared that the annexation of texas as a slave state would lead to political problems within the union. c. they thought that texas was too large to be admitted to the union as one state. d. they did not believe in the idea of manifest destiny.

Otto von bismarck once said that a great war might someday be caused by some damed foolish thing in the balkans was he right, explain