(Bloomberg Business) — Wal-Mart Stores, the world's largest private employer, this morning announced plans to start paying all U.S. employees at least $9 an hour. Current employees will make $10 an hour by next year, as will new employees who complete a six-month training course. For a company that's long been praised for low prices and slammed for low wages, that's a dramatic change.

It didn't come out of nowhere. Over the past three years, Wal-Mart has faced a wave of union-backed attacks – legal, political, media, and consumer pressure, anchored by the first coordinated U.S. store walkouts in the company's five-decade history. 

The campaign, spearheaded by the United Food & Commercial Workers and supported by a bevy of progressive groups, has mobilized only a sliver of the retailer's roughly 1.3 million U.S. employees. It announced that about 500 employees went on strike Thanksgiving week 2012 and hasn't shared a higher figure for any strike since. But it has outpaced past efforts, and foisted unwelcome headlines on the country's dominant employer.