The nation of Belgium, founded in 1830, was endowed with one of the most liberal constitutions in Europe and functioned as refuge for progressives from other countries.
From 1845 to 1848, Engels and Marx lived in Brussels, spending much of their time organising the city's German workers.
Shortly after their arrival, they contacted and joined the underground German Communist League.
The Communist League was the successor organisation to the old League of the Just which had been founded in 1837, but had recently disbanded.
Influenced by Wilhelm Weitling, the Communist League was an international society of proletarian revolutionaries with branches in various European cities.
The Communist League also had contacts with the underground conspiratorial organisation of Louis Auguste Blanqui.
Many of Marx's and Engels's current friends became members of the Communist League.
Old friends like Georg Friedrich Herwegh, who had worked with Marx on the Rheinsche Zeitung, Heinrich Heine, the famous poet, a young doctor by the name of Roland Daniels, Heinrich Bürgers and August Herman Ewerbeck all maintained their contacts with Marx and Engels in Brussels.
Georg Weerth, who had become a friend of Engels in England in 1843, now settled in Brussels.
Karl Wallau and Stephen Born (real name Simon Buttermilch) were both German immigrant typesetters who settled in Brussels to help Marx and Engels with their Communist League work.
Marx and Engels made many new important contacts through the Communist League.
One of the first was Wilhelm Wolff, who was soon to become one of Marx's and Engels's closest collaborators.
Others were Joseph Weydemeyer and Ferdinand Freiligrath, a famous revolutionary poet.
While most of the associates of Marx and Engels were German immigrants living in Brussels, some of their new associates were Belgians.
Phillipe Gigot, a Belgian philosopher and Victor Tedesco, a lawyer from Liège, both joined the Communist League.
Joachim Lelewel a prominent Polish historian and participant in the Polish uprising of 1830–1831 was also a frequent associate.

The Communist League commissioned Marx and Engels to write a pamphlet explaining the principles of communism.
This became Manifesto of the Communist Party, better known as the Communist Manifesto.
It was first published on 21 February 1848 and ends with the world-famous phrase: "Let the ruling classes tremble at a Communistic revolution.
The proletariat have nothing to lose but their chains.
They have a world to win ... Working Men of All Countries, Unite!"

Engels' mother wrote in a letter to him of her concerns, commenting that he had "really gone too far" and "begged" him "to proceed no further".
She continued:
You have paid more heed to other people, to strangers, and have taken no account of your mother's pleas.
God alone knows what I have felt and suffered of late.
I was trembling when I picked up the newspaper and saw therein that a warrant was out for my son's arrest.
The nation of Belgium, founded in 1830, was endowed with one of the most liberal constitutions in Europe and functioned as refuge for progressives from other countries. From 1845 to 1848, Engels and Marx lived in Brussels, spending much of their time organising the city's German workers. Shortly after their arrival, they contacted and joined the underground German Communist League. The Communist League was the successor organisation to the old League of the Just which had been founded in 1837, but had recently disbanded. Influenced by Wilhelm Weitling, the Communist League was an international society of proletarian revolutionaries with branches in various European cities. The Communist League also had contacts with the underground conspiratorial organisation of Louis Auguste Blanqui. Many of Marx's and Engels's current friends became members of the Communist League. Old friends like Georg Friedrich Herwegh, who had worked with Marx on the Rheinsche Zeitung, Heinrich Heine, the famous poet, a young doctor by the name of Roland Daniels, Heinrich Bürgers and August Herman Ewerbeck all maintained their contacts with Marx and Engels in Brussels. Georg Weerth, who had become a friend of Engels in England in 1843, now settled in Brussels. Karl Wallau and Stephen Born (real name Simon Buttermilch) were both German immigrant typesetters who settled in Brussels to help Marx and Engels with their Communist League work. Marx and Engels made many new important contacts through the Communist League. One of the first was Wilhelm Wolff, who was soon to become one of Marx's and Engels's closest collaborators. Others were Joseph Weydemeyer and Ferdinand Freiligrath, a famous revolutionary poet. While most of the associates of Marx and Engels were German immigrants living in Brussels, some of their new associates were Belgians. Phillipe Gigot, a Belgian philosopher and Victor Tedesco, a lawyer from Liège, both joined the Communist League. Joachim Lelewel a prominent Polish historian and participant in the Polish uprising of 1830–1831 was also a frequent associate.
The Communist League commissioned Marx and Engels to write a pamphlet explaining the principles of communism. This became Manifesto of the Communist Party, better known as the Communist Manifesto. It was first published on 21 February 1848 and ends with the world-famous phrase: "Let the ruling classes tremble at a Communistic revolution. The proletariat have nothing to lose but their chains. They have a world to win ... Working Men of All Countries, Unite!"
Engels' mother wrote in a letter to him of her concerns, commenting that he had "really gone too far" and "begged" him "to proceed no further". She continued:
You have paid more heed to other people, to strangers, and have taken no account of your mother's pleas. God alone knows what I have felt and suffered of late. I was trembling when I picked up the newspaper and saw therein that a warrant was out for my son's arrest.
Friedrich Engels