Complexi sunt autem hoc foedere rex quidem duces Stolpensium et Masoviorum. Magister vero Sigismundum regem Ungarorum, si is vellet, interim autem ne ei Polonus bellum faceret.
Too many subjects? Or is there a missing conjunction like et to denote a double subject (king and commanders)? btw I guess a complementary infinitive after vellet is understood here? Do you require more context?