This is precisely how wars get started, an act of aggression by one side followed by an act of retaliation by the other, tit for tat until someone gets nuked. Patrick Cockburn reports that Iran's capture of 15 British marines and sailors was a direct response to a botched U.S. operation in January, when the military snatched 5 Iranians in Arbil -- identified as members of a Revolutionary Guard, or Pasdaran, unit -- who were suspected of arming insurgents. (These men are still being held.) Cockburn reports that U.S. forces were actually after two senior Iranian security officials, Mohammed Jafari, the deputy head of the Iranian National Security Council, and General Minojahar Frouzanda, the Pasdaran's intel chief. At the time, both officials were in Iraq on official business, meeting with Iraqi President Jalal Talabani and Massoud Barzani, the President of the Kurdistan Regional Government.
The attempt by the US to seize the two high-ranking Iranian security officers openly meeting with Iraqi leaders is somewhat as if Iran had tried to kidnap the heads of the CIA and MI6 while they were on an official visit to a country neighbouring Iran, such as Pakistan or Afghanistan. There is no doubt that Iran believes that Mr Jafari and Mr Frouzanda were targeted by the Americans.
...The abortive Arbil raid provoked a dangerous escalation in the confrontation between the U.S. and Iran which ultimately led to the capture of the 15 British sailors and Marines - apparently considered a more vulnerable coalition target than their American comrades.
Cockburn also reports that the official rationale for grabbing the Iranians in January, hours after President Bush went public with the accusation that Iran is equipping insurgents, doesn't quite add up.
US officials in Washington subsequently claimed that the five Iranian officials they did seize, who have not been seen since, were "suspected of being closely tied to activities targeting Iraq and coalition forces". This explanation never made much sense. No member of the US-led coalition has been killed in Arbil and there were no Sunni-Arab insurgents or Shia militiamen there.