Numbers are the answer. Whenever I get beat beat as USF it's because the opposing player kept up numbers in the early game to compete. That extra manpower advantage is there to help ostheer compete with USF.
something like this:
gren>MG>MG>gren>gren>gren>tech up
or
MG>pio>MG>MG>T2>Pgren>Pgren>Pgren
So basically:
1. Use the numbers advantage early on to your benefit
2. Don't let MGs sit alone protecting areas, your infantry NEEDS them to support them or they will lose
3. Don't send out units alone, they will beat you in 1v1 battles
4. Don't leave your units in the same place after an engagement, they will remember where it is and they will flank it
5. Layer your defenses, they will smoke individual MGs
6. Be prepared to pull back, even if they use a lot of smoke it's worthless if you just pull back and make them start the assault again from the start
7. Don't charge open ground with your Pgrens, flank or use smoke
8. Don't let them take the buildings!
9. Lone pgrens are useless, 2 or more together at all times
To sum it up, you basically have to keep your units together and your defenses active and ready to move back. USF is really good at keeping their assaults moving, and will likely be able to push through any single line of defenses they meet. But if keep moving and drain their infantry as they attack they bleed quickly. Then you just counterattack when they are weak.
If you are able to hold off 2 assaults back to back early on, they just won't be able to keep up manpower wise. Letting you expand your defense and close them in.
Crap, I screwed up the title. 
The reason it was viable was because there where different things US could do. Thus axis didn't prepare for rifle spam every game. If you took away their smoke there wouldn't be anything stopping players from building multiple MGs and kubels to shut down rifle spam. And there is nothing else USF can do. 
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Omega_Warrior
Freedom.Kite(?)
Relic Entertainment
보드카 중대 
VonManteuffel 
