The American Navy’s Fifth Fleet, which patrols the region from its base in Bahrain, has vowed to stop any menace to maritime traffic.
American and Iranian forces clashed in the strait repeatedly in 1988 after an Iranian mine damaged a Navy frigate.
Iran and Iraq have grown increasingly aligned in the years since Saddam Hussein’s ouster and the subsequent rise of a Shiite-led government.
Once fierce adversaries, the most recent national elections saw Iraqi parties with links to the paramilitary Popular Mobilization Forces — an umbrella organization of about 50 paramilitary groups, many with ties to Tehran, which successfully fought against the Islamic State — gain even more power.
In quick succession, it then shored up defenses and evacuated personnel from the embassy in Baghdad, the Iraqi capital.
But the Trump administration has not provided specific details about the supposed threat from Iran, and allies in Europe and the region are skeptical given the history of faulty intelligence that led to the 2003 invasion of Iraq, led by the United States.
Occasionally, those tensions have escalated into violence.
The recent American assertion that Iran poses an immediate threat has raised fears that the two nations have pushed closer to the brink.
Tensions have risen steadily since the beginning of the Trump administration.
President Trump pulled the United States out of the nuclear deal last year, imposed oppressive sanctions, moved to cut off Iran’s oil exports and designated an Iranian military unit as a terror organization.