Continue the previous, the following figure shows US gross debts against gross domestic product (GDP) in current dollars. 
The same figure in log scale, better showing the trend. Since the great depression, The US GDP increases exponentially at a almost constant rate over the past eighty years. Over the short course, the US gross debts overwhelms GDP at one time around 1945-1947, perhaps due to the war. However, it was fortunate that the debts increased (or even decreased in some years) much slower. The question to ask today is, do we still have the same driving force for the GDP growth while resisting the temptation of more borrowings. 
Here shows the percentage of US debts to GDP. The historical highs are during the world war II and post war period. The ratio drops gradually and stables at national lows around 31% in 1970′s. It grows again in 1980′s and drops at 1995. After 2001, it soars again.
References:
[1] GDP data is from Department of Commerce, http://www.bea.gov/national/index.htm#gdp
[2] United States Department of the Treasury, http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt.htm
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