Brock Nelson finally scored for the U.S. before the end of the second, but a pair in the third by captain Alexei Kalyuzhny, around his two-minute penalty for hooking, sealed Belarus’ historic day.
"We had the momentum early and they pushed back through work and discipline," U.S. coach Todd Richards said. "They turned the tide and put us on our heels."
Belarus coach Dave Lewis agreed: "We did not settle into the game until the second period."
Finland got an early break against Slovenia with Leo Komarov and Petri Kontiola scoring in a span of 41 seconds in the first period. Aleksander Barkov and Joonas Kemppainen scored in the second for their third victory. Goalie Pekka Rinne stopped 13 shots for his third straight shutout at the worlds.
Oscar Moller netted two to lift Sweden to a hard-fought victory over Germany.
Defenseman John Klinberg scored in the third to restore the Swedish lead at 3-2, and Moller added his second goal before Matthias Plachta reduced the final margin to 4-3 with 5:21 to go.
Earlier, Germany tied the game on goals by Marcus Kink and Nicolas Krammer, after Moller and Joakim Lindstrom gave the Swedes a 2-0 lead.