Complex sentence: The 1893 treaty established a twenty-five-kilometer-wide demilitarized zone on the Right Bank, making it impossible for the Siamese to control so it became a haven for lawless characters from both sides of the river.
Split into simpler sentences: The 1893 treaty called for a twenty-five-kilometer-wide demilitarized zone along the Right Bank, which made Siamese control impossible. It soon became a haven for lawless characters from both banks of the river.