It broke away from a kingdom originally controlled by Seleucus I Nicator, and its ruler Menander notably converted to Buddhism.  
Hugh George Rawlinson has written a recent history on this polity, which was governed by Stasanor prior to its independence.  That independence occurred after it refused to pay tribute to Antiochus II, along with its sister breakaway state of Sogdiana.  The so-called "Northern Kingdom" of this polity was last ruled by Heliocles, who succeeded Phraates II of this kingdom after the Saka invaded it.  Following this kingdom's decline, the Sassanids and others came to control a polity in its same general area called Tokharistan.  It broke away from a kingdom originally controlled by Seleucus I Nicator, and its ruler Menander notably converted to Buddhism.  For 10 points, name this Greek kingdom set up in modern day ? Afghanistan following the death of Alexander the Great.
Hugh George Rawlinson has written a recent history on this polity, which was governed by Stasanor prior to its independence.  
Kingdom of {Bactria} (or {Bactriana} or {Zariaspa})