The expedition consisted of about 650 men, over 500 sailors and 100 marines, as well as five warships: Colorado, Alaska, Palos, Monocacy, and Benicia. Embarked aboard Colorado was Rear Admiral John Rodgers, also Frederick F. Low, the United States Ambassador to China. The Korean forces, known as "Tiger Hunters", were led by General Eo Jae-yeon (Hangul; 어재연 Hanja; 魚在淵).
The Americans safely made contact with the Korean inhabitants, described as "people wearing white clothes". When they inquired about the SS General Shermanincident, the Koreans were initially reluctant to discuss the topic, ostensibly to avoid having to pay any recompense. The Americans consequently let the Koreans know that their fleet would be exploring the area, and that they meant no harm. This gesture was misinterpreted; Korean policy at the time prohibited foreign ships from sailing on the Han River, as it led directly to the capital city of Hanyang, modern day Seoul. On 1 June the Korean fortress fired at the U.S. fleet as they sailed up the Ganghwa Straits, which leads to the river. The U.S. forces were not badly damaged, due "to the bad gunnery of the Coreans, whose fire, although very hot for the fifteen minutes in which they maintained it, was ill-directed, and consequently without effect." Along with the cannons being arranged in rows, one tier above another on the hill-side, and fired by a train of powder." The U.S. demanded an apology within 10 days; there was no response so Rodgers decided on a punitive assault on the forts.
On 10 June, the Americans attacked the lightly defended Choji Garrison on Ganghwa, along the Salee River. The Koreans were armed with severely outdated weapons, such as matchlock muskets. After they were quickly overrun, the Americans moved onto their next objective, the Deokjin Garrison. The poorly armed Korean forces were kept from effective range by American 12-pound howitzers. The American troops continued on towards the next objective, Deokjin Fort, which they found abandoned. The sailors and marines quickly dismantled this fortress and continued to Gwangseong Garrison, a citadel. By this time, Korean forces had regrouped there. Along the way, some Korean units tried to flank the US forces, but were beaten off again due to the strategic placement of artillery on two hills.
McKee was the first to make it into the citadel, and was fatally wounded by a shot to the groin. After him came Commander Winfield Scott Schley. Schley shot the Korean soldier who killed McKee. The flag of the Korean commander, General Eo Jae-yŏn, called the "Sujagi" by Koreans, was captured by Corporal Charles Brown of Colorado's guard and Private Hugh Purvis ofAlaska's guard. General Eo was killed by Private James Dougherty. While serving as the color bearer for Colorado's crew and Marines, Colorado Carpenter Cyrus Hayden planted the US flag on the ramparts under heavy enemy fire. Corporal Brown, Privates Dougherty and Purvis, and Carpenter Hayden received the Medal of Honor.The fighting lasted fifteen minutes. In the end, 243 Koreans were counted dead in the forts and three Americans were also killed in the fighting. The American casualties were McKee, Seaman Seth Allen, and U.S. Marine Corps Private Denis Hanrahan, 10 Americans were wounded; 20 Koreans were captured, several of whom were wounded. Five Korean forts were taken in total, with dozens of various small cannon. The Korean deputy commander was among the wounded who were captured. The US hoped to use the captives as a bargaining chip to meet with local officials, but the Koreans refused, calling the captives cowards and "Low was told that he was welcome to keep the wounded prisoners".
Following the military operations of 10-12 June, the United States Asiatic Squadron stayed at anchorage off Jakyak Island until 3 July, when they left for China.
Nine sailors and six Marines were awarded the Medal of Honor, the first for actions in a foreign conflict.
From April-May 1882, the United States and Korea negotiated and approved a 14-article treaty. The treaty established mutual friendship and mutual assistance in case of attack; and also addressed such specific matters as extraterritorial rights for American citizens in Korea and most favored nation trade status.
The treaty remained in effect until the annexation of Korea in 1910.