The National Anticorruption Bureau of Ukraine has long been a battleground in Ukrainian politics, where graft is rifeKYIV, Ukraine—Youthful protesters at the biggest demonstration here since Russia’s 2022 invasion were championing a cause that, at first glance, seemed unusual in a country known for its revolutions: the power of a law-enforcement agency.In a country that has spent more than three decades of its independence fighting the dual scourges of Russian aggression and crippling corruption, the establishment of the agency under U.S. pressure in 2015 was seen as a watershed that could boost Ukraine’s economy and accelerate its integration with the...