First, this is a geopolitical impossibility, which is why it's silly to ask this question. However, it's an interesting thought game to distinguish U.S. military capability from that of the rest of the world. Second, as for the economic conditions, the depression resulting from the loss of trade might be offset due to our massive capability to sustain massive industrial efforts. However, it wouldn't be sustainable for a long period of time. Furthermore, a good deal of our military tech is made out of materials present en masse in China. Meaning, we would be a distinct disadvantage in terms of continually producing high tech weapons our doctrine relies upon. Third, currently, the United States has the most power Navy in the world; more powerful than the next 10 navies combined. Our naval capacity would prevent any opponent from reaching. The only exception is Mexcio and South America, but they lack the logistical capability to do it. My guess is that it would take 1 to 2 years to mass the naval logistical powers to attack and another 2 years to take us over.