I'm a little late, but better late...
3 seconds sound clip from the Gone with the Wind (1939) movie soundboard.
You can hear this line at 01:28:37 in the Blu-ray version of the movie.
Quote context
[...]
- I'm going, my dear, to join the army.
- You're joking. I could kill you for scaring me so.
- I'm very serious. I'm going to join up with our brave lads in gray.
- I'm a little late, but better late...
- How could you do this to me, and why should you go now that, after it's all over, and I need you?
- Why? Maybe it's because I've always had a weakness for lost causes once they're really lost.
[...]