Complex sentence: Sebol, then regarded as a strategic point and route through Cancuén river, which communicated with Petén through the Usumacinta River on the border with Mexico and the only road that existed was a dirt one built by President Lázaro Chacón in 1928.
Split into simpler sentences: Sebol, then regarded as a strategic point and route through Cancuén river, which communicated with Petén through the Usumacinta River on the border with Mexico. The only road that existed was a dirt one built by President Lázaro Chacón in 1928.