The US has bilateral tax agreements with certain countries to avoid double taxation. So if you happen to be in one of these countries, you will normally pay taxes to your host country and if those are greater than the taxes owed to the US (which in many cases they will be), your US tax will be zero. You still have to file the US tax return and you still need to pay taxes on income derived in the US (such as dividends and interest) as that can not be eliminated by foreign taxes.