Complex sentence: After the invasion of South Korea by forces from the North on, the resulting war between the two Koreas ended with an Armistice Agreement, but the border between the two nations remains the most heavily fortified in the world.
Split into simpler sentences: After the invasion of South Korea by forces from the North on, the resulting war between the two Koreas ended with an Armistice Agreement. The border between the two nations remains the most heavily fortified in the world.