The very clear and very simple conversation Robb Stark should have had with Ser Rodrick before heading south.

Robb: "I want you to keep five hundred men manning the walls of Winterfell. At all times. Understand?

Ser Rodrick: "Yes, I understand."

Robb: "Good, now, how many men do you take from Winterfell to go off and rescue a bannerman in trouble from an attack anywhere else in the North?"

Ser Rodrick: "I suppose I'd take all of th-"

Robb: "No, that was a trick question. You're not taking any men from Winterfell. No. Taking. Men. From. Winterfell."

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.