Which word means the act of withdrawing from a country and forming your own government?
a. secession
b. nullify
c. paradox
d. sectionalism

The answer is secession or letter A.  The best example of this was seen during the American Civil War.  One of the reasons why this war began was because many Southern states left the Union because the feared the election of Abraham Lincoln would threaten their rights particularly when it comes to slavery

Both countries had to aprove annexation.