In 1848, Engels and Marx published the epoch-making "Communist Manifesto" together. After the failure of the revolution in 1848, he began to summarize the revolutionary experience and wrote "Revolution and Counterrevolution in Germany", "German Peasant War" and other works. In 1850 he returned to England.

The main reason why Engels engaged in business activities in the UK for a long time was to financially support Marx in completing the writing of Capital. Marx understood this very well and was deeply moved. Marx wrote to Engels, "My conscience often feels as heavy as a nightmare, because your outstanding talents are mainly wasted on doing business for me, and you let them go to waste, and you have to share all my trivial things." "

Engels, like Marx, independently engaged in a wide variety of scientific research and writing activities in order to lay the theory of scientific socialism on a solid scientific basis. He has conducted extensive and in-depth research on everything from natural science to military science, philosophy, economics, history, and literature and art, and has systematically elaborated on Marxist philosophy, political economy, and scientific socialism. His important works include: "Anti-Dühring", "Ludwig Feuerbach and the End of German Classical Philosophy", "Natural Dialectics", "The Origin of Family, Private Ownership and the State", etc.

After Marx’s death in 1883, Engels suspended his research work and concentrated on organizing, supplementing, and publishing the second and third volumes of Capital. It took him twelve years to complete this work, but unfortunately the "Theory of Surplus Value", which was to be published as the fourth volume of "Das Kapital", was not completed (this work was completed by Karl Kautsky, but he was Marx's original work has been extensively edited). From the "Communist Manifesto" to "Das Kapital" and even all aspects of the theory of scientific communism, the efforts of Marx and Engels were poured into it. The reason why this theory is named after Marx is because Engels said humbly: "Marx stood higher, saw further, and observed more and faster than all of us. Marx was a genius, and we At most, he is a master. Without Marx, our theory would be far from what it is now. Therefore, it is only fair that this theory is named after him."

In addition to engaging in heavy theoretical work, Engels was also very important. Pay attention to and care about the revolutionary practical activities of international communism. Together with Marx he led the First International. After Marx's death, he independently took on the leadership of the international workers' movement. He provided guidance to workers' parties in various countries through correspondence, published articles and private conversations. In 1889, under the direct leadership of Sigers, socialists from various countries established the Second International in Paris. In 1893, the seventy-two-year-old Engels personally attended the Third Congress of the Second International in Zurich. The delegates welcomed and saluted him with a storm of applause. Engels delivered a brief speech at the meeting, reminding the proletariat of various countries not to overestimate parliamentarism and political reforms in their struggles. Engels died in London on August 5, 1895. According to his wishes during his lifetime, his ashes were disposed of in the rolling sea.