The promise that Yuan Shikai broke to Sun Yat-sen was: He promised to continue forming a democracy but instead returned China to an imperial government.
Initially, Yuan Shikai had been seen as a potential leader who could help stabilize China following the fall of the Qing dynasty. However, after gaining power, he became more autocratic and attempted to establish himself as emperor, undermining efforts for democratic governance. This move led to significant political turmoil in China.