If the Assad regime begins to crumble, will Iran send in its own troops to prop it up?

It's unknown to what lengths Iran will go to support its ally, but statements coming from Tehran hint that we must entertain the possibility that Iran will intervene militarily if it feels it has no other choice.

Telegraph:

Iran has accused Turkey and Gulf countries of arming the rebel opposition in Syria, in collusion with the United States and Israel, to overthrow the Assad regime, Tehran's key regional ally.

Saeed Jalili, the head of Iran's Supreme National Security Council, appeared with the Syrian leader in Damascus on Tuesday to pledge Iran's support for Assad regime.

"What is happening in Syria is not an internal issue but a conflict between the axis of resistance on one hand, and the regional and global enemies of this axis on the other," he said.

Assad appeared on television for the first time in two weeks in footage showing him meeting Mr Jalili and vowed to cleanse Syria of "terrorists," as his troops stepped up air and artillery bombardments on rebels in Aleppo, Syria's largest city.

"The Syrian people and their government are determined to purge the country of terrorists and to fight the terrorists without respite," he said, according to state news agency SANA.

[...]

Tehran, which has voiced growing criticism of support by the United States, Turkey, Saudi Arabia and Qatar for the rebels, also sent its foreign minister to Ankara and a letter to Washington holding them responsible for the fate of 48 kidnapped Iranians.

Iranian state media quoted Mr Jalili as saying Tehran "believes in national dialogue between all domestic groups to be the solution, and believes foreign solutions are not helpful."

Note the use of the phrase "not an internal issue." Clearly, Iran believes that Syria is under attack from outside which may justify, in its own eyes, sending military support to assist Assad in the civil war.

The regime is off balance but not in any immediate danger of collapsing. But if mass defections from the army begin to affect Assad's ability to fight his enemies, all bets are off and intervention by Iran is possible.