Engels decided to return to Germany in 1844.
On the way, he stopped in Paris to meet Karl Marx, with whom he had an earlier correspondence.
Marx had been living in Paris since late October 1843, after the Rheinische Zeitung was banned in March 1843 by Prussian governmental authorities.
Prior to meeting Marx, Engels had become established as a fully developed materialist and scientific socialist, independent of Marx's philosophical development.

In Paris, Marx was publishing the Deutsch–Französische Jahrbücher.
Engels met Marx for a second time at the Café de la Régence on the Place du Palais, 28 August 1844.
The two quickly became close friends and remained so their entire lives.
Marx had read and was impressed by Engels's articles on The Condition of the Working Class in England in which he had written "A class which bears all the disadvantages of the social order without enjoying its advantages … Who can demand that such a class respect this social order?"
Marx adopted Engels's idea that the working class would lead the revolution against the bourgeoisie as society advanced toward socialism, and incorporated this as part of his own philosophy.

Engels stayed in Paris to help Marx write The Holy Family.
It was an attack on the Young Hegelians and the Bauer brothers, and was published in late February 1845.
Engels's earliest contribution to Marx's work was writing for the Deutsch–Französische Jahrbücher, edited by both Marx and Arnold Ruge, in Paris in 1844.

During this time in Paris, both Marx and Engels began their association with and then joined the secret revolutionary society called the League of the Just.
The League of the Just had been formed in 1837 in France to promote an egalitarian society through the overthrow of the existing governments.
In 1839, the League of the Just participated in the 1839 rebellion fomented by the French utopian revolutionary socialist, Louis Auguste Blanqui.

However, as Ruge remained a Young Hegelian in his belief, Marx and Ruge soon split and Ruge left the Deutsch–Französische Jahrbücher Nonetheless, following the split, Marx remained friendly enough with Ruge that he sent Ruge a warning on 15 January 1845 that the Paris police were going to execute orders against him, Marx and others at the Deutsch–Französische Jahrbücher requiring all to leave Paris within 24 hours.
Marx himself was expelled from Paris by French authorities on 3 February 1845 and settled in Brussels with his wife and one daughter.
Having left Paris on 6 September 1844, Engels returned to his home in Barmen, Germany, to work on his The Condition of the Working Class in England, which was published in late May 1845.
Even before the publication of his book, Engels moved to Brussels in late April 1845, to collaborate with Marx on another book, German Ideology.
While living in Barmen, Engels began making contact with Socialists in the Rhineland to raise money for Marx's publication efforts in Brussels.
However, these contacts became more important as both Marx and Engels began political organizing for the Social Democratic Workers' Party of Germany.
Engels decided to return to Germany in 1844. On the way, he stopped in Paris to meet Karl Marx, with whom he had an earlier correspondence. Marx had been living in Paris since late October 1843, after the Rheinische Zeitung was banned in March 1843 by Prussian governmental authorities. Prior to meeting Marx, Engels had become established as a fully developed materialist and scientific socialist, independent of Marx's philosophical development.
In Paris, Marx was publishing the Deutsch–Französische Jahrbücher. Engels met Marx for a second time at the Café de la Régence on the Place du Palais, 28 August 1844. The two quickly became close friends and remained so their entire lives. Marx had read and was impressed by Engels's articles on The Condition of the Working Class in England in which he had written "A class which bears all the disadvantages of the social order without enjoying its advantages … Who can demand that such a class respect this social order?" Marx adopted Engels's idea that the working class would lead the revolution against the bourgeoisie as society advanced toward socialism, and incorporated this as part of his own philosophy. 
Engels stayed in Paris to help Marx write The Holy Family. It was an attack on the Young Hegelians and the Bauer brothers, and was published in late February 1845. Engels's earliest contribution to Marx's work was writing for the Deutsch–Französische Jahrbücher, edited by both Marx and Arnold Ruge, in Paris in 1844.
During this time in Paris, both Marx and Engels began their association with and then joined the secret revolutionary society called the League of the Just. The League of the Just had been formed in 1837 in France to promote an egalitarian society through the overthrow of the existing governments. In 1839, the League of the Just participated in the 1839 rebellion fomented by the French utopian revolutionary socialist, Louis Auguste Blanqui.
However, as Ruge remained a Young Hegelian in his belief, Marx and Ruge soon split and Ruge left the Deutsch–Französische Jahrbücher Nonetheless, following the split, Marx remained friendly enough with Ruge that he sent Ruge a warning on 15 January 1845 that the Paris police were going to execute orders against him, Marx and others at the Deutsch–Französische Jahrbücher requiring all to leave Paris within 24 hours. Marx himself was expelled from Paris by French authorities on 3 February 1845 and settled in Brussels with his wife and one daughter. Having left Paris on 6 September 1844, Engels returned to his home in Barmen, Germany, to work on his The Condition of the Working Class in England, which was published in late May 1845. Even before the publication of his book, Engels moved to Brussels in late April 1845, to collaborate with Marx on another book, German Ideology. While living in Barmen, Engels began making contact with Socialists in the Rhineland to raise money for Marx's publication efforts in Brussels. However, these contacts became more important as both Marx and Engels began political organizing for the Social Democratic Workers' Party of Germany.
Friedrich Engels