In an era still haunted by the Franco-Prussian War, d’Estournelles strove to ease tension between France and both Britain and Germany. As a founding member of the IPU he urged nations to sign arbitration treaties, advocating courts over guns. When the 1904 Entente Cordiale was concluded, he secured parliamentary backing and promoted peaceful settlement of colonial disputes. He also maintained contacts with German deputies and used cultural events to reduce hostility. His work preserved valuable channels of dialogue in pre-WWI Europe.

As a senator, d’Estournelles submitted a 1903 report entitled "Établissement d'un Tribunal Arbitral," proposing compulsory referral clauses to the PCA. He toured St Petersburg, London and Berlin, accumulating case-specific arbitration agreements on fisheries, trade tariffs and more between Britain-France and France-Germany, demonstrating the method’s practicality. In his 1905 Chemnitz lecture he argued that ‘mutual dependence equals security,’ influencing economist Norman Angell. He also ran the "Concours de la Paix," a peace essay contest for students, to shape public opinion. The combination of grassroots outreach and institutional design boosted his reputation.

During the 1904 Anglo-French colonial settlement d’Estournelles led an amendment inserting an ICSID-like arbitration clause into the ancillary protocol during parliamentary ratification. In Franco-German relations he engineered the successful Baden Arbitration (1908), using Reichsgericht precedents to bypass jurisdictional conflicts. His ‘diplomatie parlementaire’ is regarded as a precursor to modern multi-track negotiations. As a media strategy he published the "Correspondance de la Paix," summarising arbitration cases in mainstream newspapers to educate the public. His network later provided intellectual groundwork for the Briand-Stresemann era of Franco-German rapprochement.