The Space Wolves never forget a grudge and have had a millennia long feud with the Dark Angels Space Marine Chapter and the Thousand Sons Chaos Space Marine Legion. A Space Wolf will hit on a roll of 3+, regardless of the model's respective Weapon Skills, when fighting in close combat against troops from either of these formations. However, Dark Angels and Thousand Sons opponents will reciprocate and will also hit on a 3+! You should determine which models in a Chaos army are Thousand Sons at the start of the battle.
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): Space Wolves may not hit Dark Angels vehicles (other than Dreadnoughts) on a 3+ in close combat under Blood Feud — the 3+ only applies when fighting something with a Weapon Skill attribute.