Following years of violent conflicts across Europe, Passy joined the peace movement in the 1850s, working with several notable activists and writers to develop journals, articles, and educational curricula.

[4]: 37 He was unable to secure a full-time position in education; he refused to swear the mandatory oath of loyalty to French monarch Napoleon III, believing his rule to be illegitimate.

[3]: 22  Bastiat held the belief that the conscription and high tax which often accompanied militarism had a largely negative effect on the poor, and Passy further developed these ideas on class conflict throughout his work.

[4]: 35  Years of violent discontent in Italy, Poland, and Austria and Prussia led to calls for a Europe federation from prominent liberals and socialists: Pierre-Joseph Proudhon, Émile de Girardin, Passy, and Michel Chevalier were all advocates of this idea.

In 1859, Passy condemned the idea that military action could be a solution to political issues, suggesting instead that Europe should have a "permanent congress to oversee the general interests of humanity" and an international police force.

[3]: 32 In April 1867, the Paris newspaper Le Temps published three letters attacking the actions of the French concerning Luxembourg, the third of which was written by Passy.

[3]: 33–4 In May 1867,[3]: 34  Passy and Chevalier received permission to organize the Ligue Internationale et Permanente de la Paix (International and Permanent League of Peace).

This differed greatly with the ideas of previous conservatives like Friedrich von Gentz, whose anti-war stance was concerned with maintaining the status quo.

He returned to Paris and attempted to convince the British and American embassies to provide neutral intervention in the conflict, even considering travelling by hot air balloon to the Prussian king himself.

[3]: 34 After the Ligue's collapse following the Franco-Prussian War, peace activism in Europe gained a rejuvenation after the successful arbitrations between Britain and the United States in Geneva.

Daniël van Eyk, Philip Johannes Bachiene, and Samuel Baart de la Faille founded a Dutch group on the ideas behind Passy's Ligue in 1871, and Masonic lodges began to undertake peace projects.

[3]: 47–8 Noticing the growth and popularity of the peace movement, members of the Société arranged a congress at the 1878 Paris Exposition, however they warned attendees not to raise "unpleasant" and provocative issues.

[4]: 45  He returned to the issue in December 1885, denouncing the colonialist actions of France amid the "remote prospect of any commercial results" coming from the conflict.

[15] In 1887, Passy and British MP William Randal Cremer petitioned their respective parliaments to support arbitration treaties between their country and the United States.

This meeting formed the first Inter-parliamentary Conference (later the Inter-parliamentary Union) in 1889, attended by prominent politicians like Léon Bourgeois and Jean Jaures, with Passy serving as president[4]: 50–1 Passy contributed to several different political magazines, including the feminist Revue de Morale Sociale (Review of Social Morale) and the literary-political Revue Politique et Littéraire (Political and Literary Review).

[4]: 55  In his application to the Académie, Passy avoided using the word "peace" and instead wrote: My writings and lectures have been ceaselessly devoted to the study and explanations of the principal problems of public and private morality; that this ... has been carried out not without difficulty nor without sacrifices [but] I have been able to exercise salutary influence on spirits and hearts, sometimes very decisively.

[4]: 53 In December 1901, Passy was awarded half of the first Nobel Peace Prize, which was split with Henry Dunant, the founder of the Red Cross, and each received over 100,000 francs.

Instead, Passy wrote an article to be released posthumously, criticising Alfred Nobel's executors for using his money to create foundations he did not intend, and suggesting that the award could weaken the peace movement by attracting disingenuous money-seekers instead of peace-seekers.

[3]: 167–8  A year later, he attended the 15th Universal Peace Congress in Milan, alongside delegates from across Europe and the United States like Felix Moscheles and Bertha von Suttner.

[4]: 41 In 1870, Pope Pius IX's First Vatican Council issued the Pastor aeternus, which legitimised Papal infallibility and solidified his word as divine.

Despite his Catholic background, he was supported by members of different denominations like the Protestant pastor Joseph Martin-Paschoud and Grand Rabbi Lazare Isidor.

[4]: 40–1 While acknowledging their attendance at peace congresses, Passy disagreed with the violence that often accompanied the labour movement, considering it to be a hindrance to peace-seeking efforts.

[4]: 46  Instead, he suggested that the citizen-soldier would be a better idea: Do not fear that the man who will be used to working every day in order to feed his wife and to raise his children is incapable of making the effort to defend them at a moment's notice.

He will have been an exact and conscientious labourer in his workshop, an honest and polite foreman, a boss concerned with the well-being and dignity of the men he employs; in other words, he will have known and fulfilled his duties each day.

[19] Passy's progressive views on European culture were influential towards his parenting: his son Paul learnt four languages as a child, yet never attended school.

[20]: 21–2 Passy and Sageret also had a daughter called Marie Louise, whose husband Louis André Paulian was in charge of the Chamber of Deputies's stenographic bureau.

[5][21] On 17 February 1912, Mathilde Paulian, the 20-year-old daughter of Marie and Louis, climbed over the railings of the Eiffel Tower observation deck and fell to her death, apparently upset over the ill health of her grandfather (Passy) and sister.

[24] Passy's brand of peace through arbitration and international co-operation continued long after his death, with activists lobbying for formalised treaties over "the rights of foreign visitors, joint access to waterways, settlement of territorial disputes".

[3]: 208  In his will, Passy expressed his independent and peaceful nature, writing: I ask my friends above all not to enrol me in any party, sect, or school in politics, or religion or science.

I hate nothing except that narrowness of spirit and that dryness of soul which because we are divided on secondary points prevents us from working together for the great causes in which we might easily unite.