Why did Sun Yat-sen allow Chinese Communists to join the Kuomintang?

Sun Yat-sen allowed members of the Chinese Communist Party (CCP) to join the Kuomintang (KMT) primarily to secure essential Soviet military and organizational aid.12

This alliance, known as the first United Front, was formalized following the Sun-Joffe Manifesto in 1923. In exchange for admitting communists into the KMT as individuals—a "bloc within" strategy—Sun received support to reorganize his party along centralized, Leninist lines and to develop a party-controlled military force.234

Sun’s decision was driven by the need to strengthen the Nationalist movement and reunify China against the threats of imperialism and militarism. While many KMT veterans opposed the move out of fear that the communists would eventually take over the party, Sun accepted the strategy to bolster his revolutionary efforts with Soviet resources and expertise, including the guidance of Comintern adviser Mikhail Borodin.2435