Seventy years after Chinese troops entered the Korean War to fight against U.S. troops, President Xi Jinping said on Friday that China will never allow its sovereignty, security and development interests to be undermined.

Any act of unilateralism, monopolism and bullying would not work, and would only lead to a dead end, Xi said in a speech at the Great Hall of the People.

“Let the world know that ‘the people of China are now organised, and are not to be trifled with,” Xi said, quoting Mao Zedong, the founding father of the People’s Republic of China.

Xi was speaking on the 70th anniversary of the deployment of Chinese troops to the Korean peninsula to help North Korea fight U.S.-led United Nations and South Korean forces during the 1950-53 war.

Xi did not directly refer to the present-day United States, whose ties with China have sunk to their lowest in decades amid disputes with the government of President Donald Trump.

The world’s two biggest economies have clashed over issues ranging from trade, technological and security rivalry to human rights and the coronavirus.

Xi also called to expedite the modernisation of the country’s defence and armed forces.

“Without a strong army, there can be no strong motherland,” he said.

North Korea went to war in 1950 with the South, which was backed by United Nations forces comprising mainly U.S. troops.

In October 1950, Chinese troops crossed the Yalu River on the border with North Korea while the Soviets provided air cover.

Stressing the geopolitical importance of North Korea, Mao said: “If the lips are gone, so will the teeth grow cold.”

The Korean War ended in an armistice in 1953, rather than a peace treaty, leaving the peninsula in a technical state of war.

Over 2 million Chinese troops were deployed.

“After arduous battles, Chinese and Korean troops, armed to their teeth, defeated their opponents, shattering the myth of the invincibility of the U.S. military, and forcing the invaders to sign the armistice agreement on July 27, 1953,” Xi said.

Earlier this week, the United States approved the potential sale of weapons systems to Taiwan with a total value of $1.8 billion, angering China.

Beijing considers Taiwan a wayward province it has vowed to bring under control, by force if necessary. Washington is required by law to provide the island with the means to defend itself.

Reuters