The United States and Britain have an obligation to respond . Ukrainian President Petro Poroshenko visited Washington on Friday . He seems intent on a peaceful settlement , write Strobe Talbott and Steven Cohen . Ukraine should continue to provide defensive forces on an expedited basis , authors say . Cohen and Pifer : Moscow has grossly violated the Minsk memorandum .
Ukrainian President Petro Poroshenko visited Washington on Friday . He seems intent on a peaceful settlement , write Strobe Talbott and Steven Cohen . Ukraine should continue to provide defensive forces on an expedited basis , authors say . Cohen and Pifer : Moscow has grossly violated the Minsk memorandum .