Complex sentence: However, when Triapitsyn began to round up and execute White Movement supporters, the Japanese garrison launched a surprise attack on 12 March 1920 that failed miserably, and resulted in the execution of the surviving garrison and the slaughter of all but 122 Japanese civilians.
Split into simpler sentences: However, when Triapitsyn began to round up and execute White Movement supporters, the Japanese intervened, launching a surprise attack on 12 March 1920. The attack failed miserably, and resulted in the execution of the surviving garrison and the slaughter of all but 122 Japanese civilians.