Scott Walker has snagged a key Romney ally, attorney and foreign-policy hand Robert C. O’Brien, for his not-yet-announced presidential campaign. A spokeswoman for Walker’s 527 group, Our American Revival, confirmed that O’Brien will serve as a foreign-policy adviser. O’Brien, sources say, decided to endorse Walker after being courted by several other Republican presidential contenders, including Texas senator Ted Cruz, former Texas governor Rick Perry, Florida senator Marco Rubio, and South Carolina senator Lindsey Graham. He is a personal friend of former Massachusetts governor Mitt Romney, a fellow Mormon who served as a key foreign-policy adviser on the governor’s 2012 presidential...