If the US sends troops to Lebanon as peacekeepers and/or as a force to disarm Hezbollah, we will see a quick escalation. Any attack (and it would be inevitable) on US forces by the Hez would be seen as a direct attack on the US by Syria and Iran. This would lead to an immediate expansion of hostilities (to include an EBO against both countries as a means of punishment). From that point on, the situation would be beyond repair.
Confirmation, front page Sunday LA Times