 Supporter of Międzymorze federation idea, as well as Prometheism, he was also a vocal opponent of polonization, arguing that Ukrainians and Belarusians living in Poland should be allowed to assimilate into Polish society at their will and speed.Author of many works, among them "Litwa i Białoruś" ("Lithuania and Belarus", 1912), "Ukraińska sprawa narodowa w jej rozwoju historycznym" ("The Ukrainian National Cause in its Historical Development", 1925), "Zarys dziejów PPS" ("A Short History of the PPS", 1925), "Józef Piłsudski, jakim go znałem" ("Józef Piłsudski, as I knew him", 1935).His daughter, Wanda Wasilewska, was a prominent pro-Soviet communist activist in the People's Republic of Poland.