Turkey's move complicates the situation, in part because it technically violates the national sovereignty of an independent state. Putting that aside, Turkey may have at least three basic strategic objectives:
Perhaps, Turkey's apparent readiness to adopt a constructive role in resolving this conflict may now help to step some of the negative repercussions it has spawned. A pragmatically-minded President Erdogan's recent moves may signal such a strategic rethink, back toward Turkey as a regional peacemaker.
From opposing the nuclear negotiations to impeaching key ministers, parliament made it increasingly difficult for Rouhani to fully realize his moderate political agenda -- until now.
If the United States acts with ill intent and makes it difficult for Iran to receive the benefits it is due under the deal, then years of diplomacy will be undone and a zero-sum mentality will once again take hold between the two countries -- with disastrous consequences for the region.