Yangtze River Crossing military campaign by CCP to defeat KMT 1949:
The Yangtze River Crossing campaign (Chinese: 渡江战役) was a military campaign launched by the People's Liberation Army to cross the Yangtze River and capture Nanjing, the capital of the Nationalist government of the Kuomintang, in the final stage of the Chinese Civil War. The campaign began at night on 20 April, and lasted until 2 June 1949, concluding after the fall of Nanjing and Shanghai to the Communist forces.

The campaign
Between 20 April and 21 April, 300,000 men from the PLA crossed from the north to the south banks of the Yangtze River.[6] Both the Second Fleet of the Republic of China Navy and the Nationalist fortress in Jiangyin soon switched sides to the Communists, allowing the PLA to penetrate through Nationalist defenses along the Yangtze.[7] As the PLA began landing on the south side of the Yangtze on 22 April and securing the beachheads, the Nationalist defense lines began to rapidly disintegrate.[6] As Nanjing was now directly threatened, Chiang ordered a scorched earth policy as the Nationalist forces retreated toward Hangzhou and Shanghai. The PLA stormed across the Jiangsu province, capturing Danyang, Changzhou and Wuxi in the process. As the Nationalist forces continued to retreat, the PLA was able to capture Nanjing by 23 April without encountering much resistance.(From Wikipedia)

Why KMT was defeated by CCP with a landslide victory:
In April 1949, because there are too many defectors including the top commander, General Dai Jong-Kwung 戴戎光) of the KMT military force of the "Jiangyin Fortress 江陰要塞", the National Army’s (KMT) defense line quickly collapsed. The Chinese People's Liberation Army crossed the Yangtze River in a large scale, and the National Army was devoid of fighting spirit.

Taiwan current situation:
Taiwan has been infiltrated by CCP or their "friends" in Taiwan for years. Temples, churches, schools, companies, institutions, local government and even the military all have more or less China-Friendly people among these entities.

There two major political coalition: pan-Green coalition represented by DPP (Democratic Progress Party)and pro-Blue coalition represented by KMT. Basically, pro-Green are pro-Taiwan for independence and pan-Blue are pro-China for unification either by force or not.

These pan-Blue lived in Taiwan would likely to switch side when CCP invading Taiwan similar to Yangtze River Crossing Campaign in 1949.