Complex sentence: During the 1980s the Syrian occupation of Lebanon grew stronger, and the OACL's alliance with Syria weakened; by 1987 the OACL was forced underground, since Muhsen Ibrahim refused to go along with the Syrian policy of opposition to PLO head Yasir Arafat.
Split into simpler sentences: During the 1980s the Syrian occupation of Lebanon grew stronger, and the OACL's alliance with Syria weakened. The coalition was defeated and by 1987 the OACL was forced underground, since Muhsen Ibrahim refused to go along with the Syrian policy of opposition to PLO chairman Yasir Arafat.