He establishes this apparent paradox, in so far as the economic problem is concerned, by showing that wealth in the economically civilized world is founded upon credit and commercial contract (these being the outgrowth of an economic interdependence due to the increasing division of labour and greatly developed communication). If credit and commercial contract are tampered with in an attempt at confiscation, the credit-dependent wealth is undermined, and its collapse involves that of the conqueror; so that if conquest is not to be self-injurious it must respect the enemy’s property, in which case it becomes economically futile. Thus the wealth of conquered territory remains in the hands of the population of such territory. When Germany annexed Alsace, no individual German secured a single mark’s worth of Alsatian property as the spoils of war. Conquest in the modern world is a process of multiplying by x, and then obtaining the original figure by dividing by x. For a modern nation to add to its territory no more adds to the wealth of the people of such nation than it would add to the wealth of Londoners if the City of London were to annex the county of Hertford.
Due to family matters he returned to England briefly in 1898, then moved to Paris to work as a sub-editor on the English language Daily Messenger, and then as a staff contributor to the newspaper Éclair. He also through this period acted as French correspondent for some American newspapers, to which he sent dispatches on the progress of the Dreyfus case. During 1905–12, he became the Paris Editor for the Daily Mail.
Angell is most widely remembered for his 1909 pamphlet, Europe's Optical Illusion, which was published the following year (and many years thereafter) as the book, The Great Illusion. (The anti-war film La Grande Illusion took its title from his pamphlet.) The thesis of the book was that the integration of the economies of European countries had grown to such a degree that war between them would be entirely futile, making militarism obsolete. This quotation from the "Synopsis" to the popular 1913 edition summarizes his basic argument.
Angell was also the designer of something called The Money Game, a visual method of teaching schoolchildren the fundamentals of Finance and banking. First published in 1928 by J. M. Dent & Sons, The Money Game, How to Play It: A New Instrument of Economic Education was both a book and a game. The bulk of the book was an essay on money and a discussion of economic theory, it also contained a summary of the game's story and an explanation of the rules.
Angell's book The Press And The Organisation of Society is cited as a source in F. R. Leavis' pamphlet Mass Civilisation and Minority Culture (1930). Vera Brittain quoted Angell's statement on "the moral obligation to be intelligent" several times in her work.
Angell was one of the principal founders of the Union of Democratic Control. He served on the Council of the Royal Institute of International Affairs, was an executive for the World Committee against War and Fascism, a member of the executive committee of the League of Nations Union, and the President of the Abyssinia Association. He was knighted in 1931 and awarded the Nobel Peace Prize in 1933.