Certainly
Boromir's view of
the Ring is greatly influenced by the weakening condition of
Gondor and he believes, at first, that the Ring would be a powerful
weapon if it were to be wielded by a great warlord. However, at the Council he is advised that the Ring is far too powerful and treacherous to be used by ANYONE save its maker - and apparently in both the book and the film, he accepts the correction.
However, as time goes on in the Quest, there can be no doubt that the Ring works upon his ambition as well as his fear for his people - although by that point, there probably isn't much difference between the two. In the book, this personal desire for the Ring is demonstrated when Boromir begins to talk to and about himself as a great War Lord through his possession of the Ring (at the confrontation with
Frodo in Parth Galen). True, he will be fighting for Gondor, but plainly, the matter has long since gone from what someone wielding the Ring can do to protect Gondor and what Boromir the Great would be able to do with the Ring of Power.
At the time that Boromir attempts to wrest the Ring from Frodo, he is completely under its power.
Gandalf tells
Denethor this very thing when the latter says that his son would have returned "bearing a great gift (the Ring)". Gandalf bluntly replies that had that been the case, "...you would not have known your son."
Boromir is hardly stupid, but he shows himself weaker than
Aragorn (who has already been offered the Ring by Frodo and refused it) or Gandalf (who had also been offered the Ring) or
Faramir (who refuses to try to take it from Frodo when the opportunity presents itself) or
Galadriel (who refuses it freely offered) and even weaker than Frodo himself who only succumbs to the Ring at the last moment after years of possessing it. Still, it would be wrong to think that Boromir's fall was the result of stupidity because it was not. Many "smart" people had fallen under the spell of the Ring including
Isildur and
Saruman - so Boromir isn't in such bad company after all.