Complex sentence: The League's first (and only) congress in Hong Kong in May 1929 was marred by factionalism that mirrored the struggle between Joseph Stalin and Nikolai Bukharin then going on in Moscow.
Split into simpler sentences: The league's first (and only) congress was held in Hong Kong in May 1929. It was marred by factionalism that mirrored the struggle between Joseph Stalin and Nikolai Bukharin then going on in Moscow.