Every day, everywhere around the world, a silent referendum is going on about the state of American power. President Obama has consistently failed that test. By demanding that Bashar Assad leave power and then letting him stay; by letting Assad cross a “red line” on chemical weapons with impunity; by talking big about ISIS (“degrade and destroy”) and doing little; by standing by as Iran expanded its power into Iraq, Syria, and Yemen, as Russia seized chunks of Ukrainian territory, and as China intimidated its neighbors to claim sovereignty over disputed island, the president has dissipated the most precious commodity in the world—American credibility.

Today comes yet another test of American resolve. Details remain in dispute, but it appears that Iranian Revolutionary Guard gunboats seized the Maersk Tigris, a container ship traversing the Persian Gulf either through international waters or through a small section of Iranian waters that it would be allowed to traverse under the international legal doctrine of “innocent passage.” Instead of allowing the ship to go on its way, the IRGC fired a shot across its bow and detained the ship along with its crew. This is a vessel flagged in the Marshall Islands, a U.S. protectorate, owned by the Maersk line (a company with substantial American operations that is headquartered in Denmark, a NATO ally), and chartered by Rickers Ship Management, the Singapore-based subsidiary of a German company (two more U.S. allies).

The Iranian action may well be an indirect response to the U.S. decision to deploy an aircraft carrier strike group in order to intimidate Iran into turning back a cargo of supply ships reportedly bringing weapons to Iranian-backed Houthi fighters in Yemen. But whatever caused the Iranian action, it is a direct threat to freedom of navigation, which the U.S. Navy has defended around the world for centuries.

In the Persian Gulf, the U.S. commitment to that doctrine led President Reagan to order U.S. Navy ships to escort tankers and protect them from Iranian attacks, precipitating a short and sharp conflict (the Tanker War of 1987-88) between the U.S. and Iran. This was the last time, incidentally, that the U.S. used force to respond to Iranian attacks and it was an unqualified success—the Iranians lost some oil platforms and boats that they had been using to harass shipping. Finally the accidental shootdown of an Iranian airliner in 1988 by the USS Vincennes (an unintended and unfortunate consequence of these operations) helped convince the Iranian leadership to end their war with Iraq.

Today the U.S. still remains committed, at least on paper, to protecting freedom of navigation in the Persian Gulf. In 2011, a 5th Fleet spokesman put it well: “The free flow of goods and services through the Strait of Hormuz is vital to regional and global prosperity. Anyone who threatens to disrupt freedom of navigation in an international strait is clearly outside the community of nations; any disruption will not be tolerated.”

Thus if the Obama administration were, in fact, to “tolerate” this disruption of the free flow of shipping it would send a dangerous signal, or to be more accurate, to reinforce a signal already sent: The U.S. lacks the will to stand up to predators in the international system, and in particular to Iran. Put another way, it would signal to the entire region that the president is so invested in reaching a deal with Iran that no Iranian misconduct—not the dropping of barrel bombs on Syrian civilians, not the takeover of Yemen, not the ethnic cleansing of Sunni communities in Iraq, and now not the seizure of a Western cargo ship—will be allowed to interfere with his objective.

The fate of the Maersk Tigris does not matter much in and of itself, but it will say much about this administration’s commitment to maintaining America’s traditional security responsibilities.