As a nation, the United States was born with a debt-to-G.D.P. ratio of about 42 percent, thanks to loans that were taken out to finance the American Revolution. In fact, throughout the nation’s history, the most common cause of increases in the debt-to-G.D.P. ratio has been the expenses associated with military conflict.

The Civil War increased the ratio from 2 percent in 1860 to 34 percent in 1865. World War I increased it from 3 percent in 1914 to 31 percent in 1919. And World War II increased it from 44 percent in 1941 to 109 percent in 1946, the highest level in history.

The second most common cause of increases in the debt-to-G.D.P. ratio has been deep economic downturns. In 1933, during the Great Depression, the ratio was 44 percent, up from 16 percent in 1929. The recent financial crisis and deep recession have had a similar effect. The debt-to-G.D.P. ratio has increased to 77 percent, from 36 percent in 2007.

SO what does President Obama mean when he talks about fiscal sustainability? He doesn’t mean running a surplus and repaying the debts that have been incurred on his watch, as people who spend more than they earn would have to do. Nor does he mean balancing the budget, as Representative Ryan suggests. Rather, the president seems to mean keeping the debt-to-G.D.P. ratio stable at this new, higher level. That is certainly what the last budget he submitted proposed to do.

Achieving this goal is much easier than balancing the budget. Because G.D.P. grows, the government debt can continue to grow as well, just not too fast. Stabilizing the debt-to-G.D.P. ratio requires that future budget deficits be smaller than they have been over the last few years, but they can still be sizable.

Yet this goal, hard to reach as it might be in the current political environment, is still too modest. The problem is that budget projections are based on forecasts, and such forecasts exclude the extreme events that have historically driven up government debt.

Military and economic catastrophes are, by their nature, unpredictable. While we can’t plan on one, prudence requires that we take their possibility into account. In normal times, when we are lucky enough to enjoy peace and prosperity, the debt-to-G.D.P. ratio shouldn’t just be stable; it should be falling. That has generally been the case throughout our history, and it should become the case again as we look forward.

The bottom line is that President Obama is right that sustainability is a reasonable benchmark for evaluating long-run fiscal policy. But the standard he applies when evaluating it appears too easy. It will leave us too vulnerable when the next catastrophe strikes.