USS Wyoming under Captain David McDougal, sailed into the strait and single-handedly engaged the US-built but poorly manned Japanese fleet.

The Shimonoseki-based Chōshū domain, under Lord Mōri, followed the order and began to take action to expel all foreigners by the date fixed, May 10 on a lunar calendar.

Openly defying the shogunate, Mōri ordered his forces to fire without warning on all foreign ships traversing Shimonoseki Strait between Honshu and Kyushu.

The American merchant steamer Pembroke, under Captain Simon Cooper, was riding at anchor outside Shimonoseki Strait when it was intercepted and unexpectedly fired upon by two European-built warships belonging to the Chōshū.

The crew of one enemy vessel taunted the frantic American seamen with the loud and unnerving cry, "Revere the Emperor and drive out the barbarians!"

The next day, June 26, the French naval dispatch steamer Kienchang was also riding at anchor outside the strait when Japanese artillery, atop the bluffs surrounding Shimonoseki, opened fire on her.

Her skipper, Captain François de Casembroot was convinced that Lord Mōri would not fire on his vessel due to the strength of his ship and longstanding relations between the Netherlands and Japan.

De Casembroot returned fire and ran the rebel gauntlet at full speed, fearful of endangering the life of the Dutch Consul General, who was aboard.

Momentarily ignoring the batteries, McDougal ordered Wyoming to continue steaming toward a bark, a steamer, and a brig at anchor off the town of Shimonoseki.

As McDougal wrote in his report to Secretary of the Navy Gideon Welles on July 23, "the punishment inflicted and in store for him will, I trust, teach him a lesson that will not soon be forgotten."