"The U.S. could weaken the currency and help exports however. That's what Japan is doing as Krugman has said."
It's important to distinguish between currency manipulation and expansionary monetary policy.
China engaged in currency manipulation and I suppose an argument could be made that the German/eurozone situation is effectively currency manipulation in Germany's favor. But Japan has simply conducted expansionary monetary policy.
You can see this in terms of the differing impacts on their respective trade balances.
Both China and Germany have had large trade surpluses. But Japan has a trade deficit, and since the latest QE was initiated, both imports and exports have grown quickly with imports growing even faster than exports.
So Japan's trade balance has actually grown worse. The benefit of expansionary monetary policy comes from increased aggregate demand, not from higher net exports. China (and perhaps Germany) has held the value of its currency low while sterilizing the effect of this policy on domestic demand.
Both policies result in lower exchange rates, but one allows domestic to demand to rise and one does not. That's why currency manipulation is "beggaring thy neighbor", and expansionary monetary policy is not.
Japan allowed inflation to rise, while China and Germany did not.