This was the first Group B game for the Americans, who are tied for first place with Canada. The Canadians downed Germany 9-3 on Wednesday. The U.S. plays Russia on Friday.
Also Thursday, Switzerland beat Latvia 7-2 in Group A.
U.S. goaltender John Gibson stopped 19 shots through two periods. He was replaced for the third by Jon Gillies, who made seven saves.
Sean Kuraly added a goal and an assist for the Americans. Goalie Marvin Cupper had 38 saves for the Germans.
The Americans were up 3-0 after the first period on goals by Kuraly, Jacob Trouba and Galchenyuk. They scored another three in the second -- Barber, Shayne Gostisbehere and Ryan Hartman. J.T. Miller then scored on a solo effort midway through the third and Seth Jones completed the rout less than two minutes later.
Russia is third in Group B after beating Slovakia in overtime on Wednesday. Germany is in last place.
Earlier in Group A, Mike Kunzle scored two goals to help give Switzerland a winning start to the tournament. Luca Boltshauser had 30 saves for Switzerland while Elvis Merzlikins had 25 for Latvia, which lost its second straight game.
Switzerland is tied for first at three points with Finland and Sweden in Group A. The Czech Republic is also in the group.
In Friday's other preliminary-round games, it's Finland vs. Czech Republic, Sweden vs. Switzerland and Canada vs. Slovakia.
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