WASHINGTON -- The United States is now plunging into a fundamental overhaul of its assistance to developing nations, demanding that applicants for a rich new source of financing prove their worthiness. Already countries from Bolivia to Bangladesh are competing to be among the winners. This month, the board of the new Millennium Challenge Account met for the first time to lay groundwork for grants that President Bush has said will total $5 billion annually by 2008. In the first year, perhaps just 15 nations will win awards. The program is ambitious. If it is fully financed, the Millennium Challenge Account...