Byeongin Persecution
The Daewongun launched a sweeping persecution of Catholics, executing nine French missionaries and thousands of believers; it became the pretext for the French expedition.

Paris Foreign Missions Society · Public domain · Source
The Daewongun had first hoped to use the French missionaries to check Russia’s advance, and when that failed he turned on the church amid yangban demands to ban it. Bishop Berneux and eight other French priests and some 8,000 believers are said to have died. Father Ridel escaped and reported the killings, bringing a French squadron to Korea.
