President Xi Jinping on Wednesday said China must be and will be reunified with Taiwan, and called for the two rivals to work together to realise the “historic task” of complete reunification.

“It is a historical conclusion drawn over 70 years of development of cross-strait relations, and a must for the great rejuvenation of the Chinese nation in the new era,” the Straits Times quoted Xi as saying in a speech at Beijing’s Great Hall of the People to commemorate the 40th anniversary of “Message to Compatriots in Taiwan”.

The Message was a policy document issued by the National People’s Congress — China’s Parliament — on January 1, 1979, the same day China and the US formally established relations after Washington broke ties with Taiwan.

Up until 1979, China had conducted routine artillery bombardment of Taiwan-controlled offshore islands.

But no peace treaty or formal end to hostilities has been signed since the detente, even as business, cultural, and personal links flourished.

In his speech, Xi sent a warning to advocates of Taiwan’s independence, including supporters of Taiwan President Tsai Ing-wen.

Tsai, who has refused to affirm the “One China” consensus reached between Taiwan and China in 1992, has warned against continued threats from China.

Xi said, “It’s a legal fact that both sides of the Strait belong to one China, and cannot be changed by anyone or any force.”

Beijing “reserves the option of taking all necessary measures” against outside forces that interfere with peaceful reunification and against Taiwan independence separatist activities, he said.

The Chinese leader also called for efforts to foster wide-ranging “democratic consultation” between representatives from both sides and deepen integrated development across the Taiwan Strait.

He pledged to further institutionalise cross-strait economic cooperation and to forge a common market.

Both sides should enhance the free flow of trade, connectivity in infrastructure, exchange of energy and resources, and shared industrial standards, Xi said.

Unification would be done under a “one country, two systems” approach that would “safeguard the interests and well-being of Taiwanese compatriots”, and Taiwan will be guaranteed lasting peace, he said.