In 1915, Sun Yat-sen married Soong Ching Ling in Japan and the couple saw each other through thick and thin.

Dismissing the provisional constitution Yuan Shikai soon put on the imperial robe and declared himself emperor. Sun Yat-sen issued a declaration denouncing Yuan, which was echoed by anti-Yuan struggles in many provinces in China. Yuan fell from power 83 days after he put himself on the throne; his successor was another feudal warlord backed by imperialism.

Sun Yat-sen went to Guangzhou in 1917 to take the post of the commander in chief of the Constitution Protecting Military Government, but without a single soldier or any arms he was elbowed out by the warlords and was forced to resign again.

In 1921, Sun Yat-sen took office as the extraordinary president of the Republic of China in Guangzhou, but was soon betrayed by Chen Jiongming, who had reached the position of commander of the land force of the government of the Republic of China through Sun's support.

This was a time of tremendous change in the world. The October Revolution in Russia had succeeded in 1917. In 1919 the May 4th Movement started in Beijing, facilitating the Chinese working class' ascension to China's political arena. Two years later, in 1921, the Chinese Communist Party was born.

Sun Yat-sen came to know the Communist Party of China (CPC). In Shanghai he met with Li Dazhao, one of the founders of the Communist Party of China, and Lin Boqu, an old member of Tong Meng Hui, who joined the Communist Party. Their talks filled Sun Yat-sen with hope and strengthened his confidence. He came to see that unless the future revolution took the Russian course as its example it was bound to fail. He also met Soviet representative Joffe, and with him put together a joint declaration on the Chinese revolution.

Soon afterwards Sun Yat-sen drew up the policies of alliance with Russia, co-operation with the Communist Party and assistance to the peasants and workers.

The Kuomintang, reformed and led by Sun, held its first national congress in 1924 and allowed members of the Communist Party to join it. This was the first phase of co-operation between the Chinese Communist Party and the Kuomintang and gave new life to the revolution. The manifesto of the congress gave a new interpretation of the Three People's Principles advanced by Sun Yat-sen in 1905. Here, nationalism was explained as achieving full equality for all nationalities within China and opposition to imperialism; democracy as allowing all common people to have democracy and not just the privileged few; and people's livelihood as equalizing land ownership and regulating capital. The three principles, then known as the New Three People's Principles, was the political foundation for the cooperation between the two parties.

Upon the death of Lenin, Sun Yat-sen wrote an elegy, calling Lenin the friend of the nation and teacher of the people.

To establish a military force for revolution, Sun Yat-sen founded the Huangpu Military Academy with the help of the Chinese Communist Party and Soviet advisers. He himself headed the academy, with Chiang Kaishek as president and Liao Zhongkai KMT representative and hoped to establish, through the academy, a revolutionary army distinct from the old landlords' army.

As China sank deeper under the exploitation and oppression of imperialist powers and disintegrated as the warlords pursued their warfare, Sun Yat-sen determined to use force to overthrow the northern warlords. In September 1924, he led an advance party of his army to Shaoguan, Guangdong Province, from where he set out on the northern expedition. He stated that the aim of the expedition was not only to overthrow the northern warlords but also the imperialist powers on which the warlords depended.

The CPC-KMT cooperation propelled the revolution forward and promoted the worker-peasant movement.

In October 1924, a coup d'etat was staged inside the northern warlord government. Feng Yuxiang, leader of the coup, invited Sun Yat-sen to Beijing. For the sake of peaceful unification of the nation and propagating revolutionary propositions, Sun Yat-sen and Soong Ching Ling travelled north.

Sun Yat-sen proposed holding a national conference with the purpose of overthrowing the warlords and imperialism, believing that to be the only way China could achieve unification and peace.

Already in poor health, Sun Yat-sen's condition worsened in Beijing and he died on March 12, 1925. On his deathbed he still repeated these words: "Peace, struggle on, save China."

In a testament he wrote before he died he said: "For 40 years I have dedicated myself to the national revolution. To ensure the liberty and equality for China, we must awaken our own people and ally ourselves with peoples of the world who treat us as equals."

His coffin was first kept in Beijing and in 1929 it was moved to Nanjing.