These two words probably represent the most common usage problem in papers, but the distinction between the words is painfully simple. "It’s" always means "it is." "Its" never does. At first glance, there seems to be an inherent inconsistency, because we usually use apostrophes to indicate possession, but certain words, for instance "its," "hers," and "yours," automatically show possession and need no apostrophes. When you write "it’s" be certain that you mean two words rather than one. Read it to yourself aloud if you have to, reading every "it’s" as "it is."
In war, a country must protect its borders.
It is understood that part of the area’s soil is of glacial origin.