Why did Iran really let the 10 US sailors go free?

A diplomatic disaster was averted after two small US Navy ships in the Persian Gulf were boarded by Iran and the 10 sailors on board detained on Tuesday.

The incident caused a minor panic in the US because of echoes of a similar event in 2007 which sparked an international crisis, but the newly opened diplomatic channels between Washington and Tehran quickly confirmed that the ships had entered Iranian waters without permission, thought to be because of a mechanical failure.

The crews were released after being held overnight.

While Iran's Islamic Revolutionary Guard Corps claimed that the sailors were released after they apologised, Secretary of State John Kerry and several other US officials responded that no apology was given, as there was nothing to apologise for.

It is thought that the dispute came to such a remarkably swift resolution because both countries worked for too long and too hard on the nuclear deal, finally reached in April last year, and have too much to lose to abandon it now.

Iran released a video from state media of two sailors saying they were sorry, but it's unclear whether they made the apology under duress.

Iran may have been particularly motivated to downplay the incident for economic reasons. As the Independent's diplomatic editor Kim Sengupta notes:

Externally, Iran is involved in wars in Iraq, Syria and Yemen. Even the lifting of the sanctions may not be quite as much of a boon as had been expected. The unfreezing of $56bn out of $100bn of assets will undoubtedly be a significant boost, but the drop in oil prices from $100 to $30 a barrel has been a massive blow. The future is unlikely to be plain sailing.

The sanctions, in place since the Islamic Revolution of 1979, are expected to be eased in a matter of days.