okay, explain why the Phillies traded away Cliff Lee? Roy Halladay was willing to sign with them for below market price, so it's not like they couldn't have afforded both.
1) Story goes that Cliff Lee wasn't interested in signing a long-term deal. He wanted to go into Free Agency after this year.
Phillies wanted an ace they could keep.
2) Phillies thought Cole Hamels was going to be lights out, or at least Roy Halladay Jr. all season, not just the last 3 months
3) What you know you need, while in the midst of a "job" is different than what you think you'll need when you prepare to start a job. Going back to point 1, Phillies thought Hamels & Halladay would be enough, with their starting 8, to win the division and beyond. After being a .500 club with the starting infield starting together only 6 times by the trade deadline, the Phillies realized they needed another pitcher. Thus enters Oswalt.
4) Over the 127 year history of Philadelphia Phillies baseball, the last 4 years are the exception, not the rule for this team.