Following three attacks on American merchant ships in the harbour of Rio de Janeiro, a bloodless naval engagement occurred between a United States Navy warship and an ironclad of Rear Admiral Saldanha da Gama's rebel fleet.

[3][4] The incidents which led to the short engagement began on 21 January 1894, when the 586 ton American bark SS Julia Rollins was fired on with rifles from a fort on Cobras Island, while heading to Gamboa.

Brazilian fire was heavy though nobody was harmed; the American crewmen were forced to take cover and maneuver their ship out of the fort's range, where they laid anchor.

Julia Rollins moved once more until finding a safe anchorage, then a boat with a few sailors was lowered and sent to inform admiral Andrew E. K. Benham, who was in command of three cruisers at Rio de Janeiro with orders to protect American interests and observe the rebel blockade of the city.

Admiral Saldanha da Gama's fleet of twenty-four ships was much stronger, as most of the rebel navy was stationed at Rio de Janeiro, though only the Trajano, the cruiser Guanabara and the gunboat Liberdade were directly involved.

[2][3][5] The affair climaxed on 29 January when Benham ordered commander Brownson to escort the American merchantmen into the wharves, where their cargoes would be unloaded, and to attack the first Brazilian ship to open fire.

Commander Brownson decided to wait for the engagement to end so he did not violate neutrality, and as soon as the armed tugs hauled off the American ships were underway.

Immediately Detroit returned fire with one of her 6-inch (150 mm) guns and a shot hit the Trajano's stern about two or three feet from the starboard bow, causing light damage.