President Trump should direct the U.S. military to re-seize an Iranian oil tanker released from British custody this weekend.

The tanker, Grace 1, now renamed Adrian Darya 1, was seized by the British Royal Navy in July for attempting to transport more than $100 million in oil to Syria. That transportation breached European Union sanctions on the Syrian regime. Unfortunately, last weekend, the British government released the tanker from its Gibraltar territory. It said Iran had provided assurances that the oil will no longer go to Assad. Maritime tracking websites show the Adrian Darya 1 now en-route to the Greek port of Kalamata.

But Iran cannot be trusted here.

A U.S. indictment against Adrian Darya 1 attests that Iran's Islamic Revolutionary Guard Corps has used the vessel to illicitly and covertly transport oil across the globe. The revenue generated by those sales has then filled IRGC coffers, enabling its conduct of hostile activity. The indictment also clarifies how Iran has sought to conceal the vessel's purpose and destination, even engaging in at-sea ship-to-ship transfers. This is sanctions-busting 101.

It is highly likely that at some point over the next few weeks the Iranians will again attempt to offload the vessel's oil in furtherance of new revenue generation. But the now-unsealed indictment means U.S. credibility is also now tied to Adrian Darya 1's future. Were the U.S. to thus allow the Adrian Darya 1 to offload its oil cargo, the IRGC would regard their success as testifying to American weakness. That cannot be allowed to happen.

This isn't about posturing, it's about holding firm to a policy that is paying dividends. The Trump administration has successfully balanced an interest in avoiding conflict with Iran with pressuring Iran's return to negotiations over its nuclear program. That requires a balancing act between avoiding unnecessary escalation while also deterring the IRGC-aligned Iranian hardliner faction. But if the IRGC sees that the U.S. is cowed by its threats not to intervene against the Adrian Darya 1, it will be encouraged to the belief that Trump is hesitant to challenge Iranian breaches of his aggression. In turn, it will also have better opportunity to undercut escalating efforts by more-moderate Iranian figures to seek a compromise with the United States.

Why, the hardliners will say, should we compromise when we can test American resolve and find it wanting?

Trump must send the U.S. Navy into action.