U.S.-Iranian tensions continue following attacks this month on a Saudi oil pipeline and United Arab Emirates tankers in the Persian Gulf. The Abraham Lincoln Carrier Strike Group and Kearsarge Amphibious Ready Group joined the U.S. Fifth Fleet to deter hostilities, or respond as required. The force consists predominantly of ships from the U.S. Atlantic Fleet, but if tensions continue, Hawaii’s Pacific Fleet may be directed to dispatch their reliefs.

America’s force concentration sends a strong warning that Tehran cannot ignore. Iran’s political leaders do not want open conflict with the U.S. but like many ideological regimes, Tehran has a two-legged power structure: the government that executes official policy; and the ideologues who hold the real power, the clerics’ Supreme Council and the Iranian Revolutionary Guard Corps (IRGC).

The IRGC has its own police, intelligence, cyberwarfare, air, ground, naval and special forces. President Hassan Rouhani probably influences but does not control the IRGC. Nominally under the clerics’ control, IRGC leaders enjoy great latitude, supported by religious hardliners who fervently believe constant conflict advances the “Islamic Revolution.” As such, they represent the wild card that American commanders and allies must always be prepared to counter.

The IRGC “advises” the proxies such as Hezbollah and Yemen’s al-Houthi that Tehran employs to minimize the risk of retaliation by providing plausible deniability.

Therein lies the challenge facing the U.S. Central Command and Fifth Fleet. Naval exercises two weeks ago in the Arabian Sea were routine, but reflected the fleet’s security concerns.

Fifth Fleet faces a diverse and multi-dimensional threat, including maritime suicide bombers. But the recent drone attack in Saudi Arabia highlights the changing nature of the threat. Drones can be purchased commercially and virtually any tech-savvy teenager can fly them into a target. They can also be employed in swarms. Iran’s naval exercises have periodically included remotely piloted vehicles (RPVs), the precursor to today’s drones, simulating attacks on shipping. The IRGC probably trained those who conducted this month’s attack.

Iranian TV has also shown IRGC combat swimmers placing limpet mines, a type of maritime explosive device, against ships’ hulls during exercises. The “swimmer attack” exercise demonstrated an intent and possible capability against moored or anchored ships.

The IRGC maintains one or two small-boat squadrons on the islands in the Strait of Hormuz to conduct “commando raids” or suicide attacks. They are backed by Boghammer squadrons equipped with light automatic cannon, machine guns and rockets. They are backed by anti-ship cruise missile batteries as well as short- and medium-range ballistic missiles; Iran’s Shahab-1 and -2 ballistic missiles, fired en mass into a constricted sea space, can pose a very real threat to naval forces.

The Islamic Republic of Iran Navy (IRIN) frigates and missile and torpedo patrol boats round out Iran’s conventional surface forces, and IRIN’s submarines add a subsurface component that cannot be ignored. Iran’s three Soviet-built Kilo-class and one indigenous Fateh-class submarines offer a significant anti-submarine warfare challenge in the Arabian Sea. Exceptionally quiet and heavily armed, the Kilos can patrol throughout the Western Indian Ocean. The IRIN’s 23 Ghadir-class midget submarines have a very limited armament and range, but their relatively small-size makes them difficult to detect and engage in shallow waters such as the Persian Gulf.

Iranian leaders recognize they cannot defeat a U.S. naval task group in a traditional naval battle and will instead use asymmetric tactics if conflict erupts. Facing an opponent optimized for long-distance combat in deep water and restricted by tight rules of engagement, Iran will employ swarm tactics to overwhelm the fleet’s defenses. Their light forces will initiate action at close range, within a few miles of the ships while missile and drone units will fire mass launches. It sounds unstoppable but in fact, such tactics require close coordination and practice to be effective.

U.S. fleets and ships drill constantly to prepare for whatever situations or threats may arise. Hawaii’s Pacific Fleet must stand sharp and ready to provide relief should U.S.-Iran tensions escalate and press the Atlantic Fleet into deeper service. No enemy, weapons system or tactic is omnipotent except against an unprepared or complacent opponent. Although conflict is unlikely, our military faces a very real threat. Presence deters aggression, but preparedness ensures deterrence.

Carl O. Schuster, a retired Navy captain and former director of operations at the U.S. Pacific Command’s Joint Intelligence Center, is an adjunct faculty member at Hawaii Pacific University; the views expressed here are his own.