Deploying Royal Marines from its Gibraltar territory at the Mediterranean Sea gateway, Britain on Thursday seized an Iranian oil tanker it says is breaching European Union sanctions against trade with Bashar Assad's Syrian regime.

This is proof of Britain's increasing alignment with America on Iran.

Britain insists it supports EU efforts to stabilize the Iran nuclear agreement. But London knew full well the fury its seizure would spark in Tehran.

Upset that its global circumnavigation (transiting the Suez Canal would have been quicker than traveling around the Cape of Good Hope!) to resupply Syria has been busted, Iran is warning that unless the tanker is released, it will seize a British tanker in retaliation. This threat should not be judged idle. Iranian hardliners are desperate to increase pressure on the EU to get it to weaken crippling U.S. sanctions. And they will regard Britain's action as a pretext to act.

The British are well aware of this, and their action here cannot be disconnected from its broader strategic environment. With a new prime minister entering office in late July and Iran now overtly breaching the nuclear accord, Britain's Iran policy is ripe for reconsideration. The fact that Assad has used Iranian oil to enable his massacre of hundreds of thousands of Syrians only consolidates British action under international law.

But the headline here is that the fragile coalition holding together the Iran nuclear agreement has suffered another blow. Iran is increasingly isolated.