Sun Chuanfang retreated to Nanjing in the aftermath of these setbacks. The Fengtian clique responded to Sun's plea for help by reinforcing Jiangsu and Anhui provinces, while increasing the number of troops in Henan in support of Wu Peifu. Two major Fengtian formations, the Shandong Army of Zhang Zongchang and the Zhili Army of Chu Yupu, crossed the Yangtze River in February 1927 to help Sun defend Nanjing and Shanghai. Following their victory in Zhejiang, Chiang Kai-shek ordered the launch of an offensive on those two cities. The Hangzhou-based eastern NRA, led by Bai Chongxi and He Yingqin, launched a two-pronged attack in mid-March. Bai's forces advanced toward Shanghai, whilst He's forces moved toward Changzhou, with the goal of severing Sun's lifeline, the Shanghai–Nanjing railway. Meanwhile, Cheng Qian's central NRA advanced toward Nanjing through Anhui province, its path opened by the defection of Sun's forces there. The remnants of Sun's forces, supported by the Shandong Army, were forced to withdraw to Shanghai proper in the face of Bai's army. He's forces quickly severed the railway link with Shanghai, while Sun was confronted with the defection of his navy, and a communist general strike in Shanghai. Intense fighting took place at Songjiang, just outside the city, but on 22 March, Bai's forces marched into Shanghai victorious. The Fengtian support operation had proven to be a "costly operational disaster" for the northern warlords, whose armies had suffered heavy casualties, forcing them to retreat north across the Yangtze. Meanwhile, the strike continued until 24 March, when Bai ordered its end. The general disorder caused by the strike is said to have resulted in the deaths of 322 people, with 2,000 wounded, contributing to KMT feelings of unease with its wayward communist allies.