Since then, as rebels have made inroads across Syria, Iraq’s minority Sunnis have become increasingly resolute, Iraqi officials and Western diplomats say.
Use the adjective resolute to describe a purposeful and determined person, someone who wants to do something very much, and won't let anything get in the way.
Of course, Obama was forced into such measures by an obstinate opposition.
obstinate is most definitely negative because it implies a kind of hard-headed determination not to change your mind even when it might be best to rethink your position.