A democrat and supporter of the Sejm, Sikorski declared his opposition to Józef Piłsudski's May coup d'état in 1926; however he remained in Lwów, refused to dispatch his forces, and played no significant role in the short struggle surrounding the coup. In 1928 he was relieved by Piłsudski of his command, and while he remained on active service, he received no other posting. That year also saw the publication of his book on the Polish–Soviet War, "Nad Wisłą i Wkrą. Studium do polsko–radzieckiej wojny 1920 roku" (At the Vistula and the Wkra Rivers: a Contribution to the Study of the Polish–Soviet War of 1920). He would spend the following years studying military theory, publishing works on military theory, history and foreign policy. His most famous work was his 1934 book "Przyszła wojnajej możliwości i charakter oraz związane z nimi zagadnienia obrony kraju" (lit. "War in the Future: Its Capacities and Character and Associated Questions of National Defense", published in English in as "Modern warfare. Its character, its problems" in 1943), in which he predicted the return of the maneuver warfare. He wrote several other books and many articles, foreseeing, among other things, the rapid militarization of Germany.