It was no mistake. Iran is deliberately holding 15 British servicemen.
It is an act of war. The United States should fully back Great Britain in any move that it may have to undertake if Iran decides to make this a hostage situation.

If the Iranians believe that seizing the British servicemen will give them leverage in the ongoing dispute with the West over their nuclear program then they are gravely mistaken. Indeed, if this situation escalates then it may very well give the West an opportunity to clean the clock - once and for all - of the Iranian nuclear enrichment sites.