Sorry to be the Party Pooper, but diplomacy and improved relations between Zarif and John Kerry were not the only, nor the biggest reason why the captured U.S sailors were released within 24 hours by Iran.
The biggest reason? The end of sanctions being so close.
The Iranian supreme leader is a realist. He is not going to ruin years of negotiations and sacrifice all the compromises he made to reach a nuclear deal so that sanctions are removed by locking horns with the U.S at this point. Not when he is so close to the finishing line.
And this is what gave the moderates in Iran leverage against the hardliners. The latter would have absolutely loved to turn this incident into something more serious as it would have boosted their “revolutionary” credentials and their defense budgets. Not to mention leveraging it to gain more political room for maneuvering inside Iranian domestic politics.
But today, it wasn’t to be. Lets see how they will react next time, after the supreme leader has received what he has wanted all along: end to sanctions. Then a simple incident like today could get more complicated. And friendly relations between Kerry and Zarif may not be enough to solve things within 24 hours, like today.