Starts to crank but stops with good battery and starter.

What is your question?

2000 Buick Park Avenue, 3.8 liter, NOT supercharged. I pulled into my driveway, changed the oil and filter and added 4 quarts of oil. And just like that, the car wohn't start. I pulled the starter and it checked out OK at auto zone. I put it back in correctly and tried to start the car. It just clicked, the solenoid, that is. So, I took the battery out and had it fully charged at auto zone. They said it had been low but was now fully charged. I put the battery back in and tried to start the car. It still won't start. It tries to start. It turns the engine just a hair and then stops. I can see the springs through the oil filler hole beginning to compress. It goes rerr-err, but not err-err-err-err-err-err-err... Even with a gas problem, shouldn't it crank continuously, so long as the starter works and the battery has stays fully charged? What would suddenly cause this and what could keep it from cranking more than just moving the belt like an inch or two?