As a fraction of GDP. You have to sharply raise your taxes and cut your expenditures, i.e., a sudden relative austerity. Austerity chokes off economic growth, thus expanding the debt to GDP ratio.
Depending on how loose government spending was before, you may or may not be able to survive it. Italy has been struggling along for quite some time trying to control its debt load, and the result has been to induce slow growth.
Italy will be forced to default on its debt in some way if its rates don't go down sharply within a year. Its long yields are actually lower than some of its short yields as of this morning - that means recession.
The math is very simple. At debt 100% of GDP, an average interest rate of 3.5% means that a country has to run a primary surplus of 3.5% of GDP - GDP growth rate to not increase debt/GDP ratio. At 100% of GDP, an average interest rate of 6.5% means that a country has to run a primary surplus of 6.5% - GDP growth rate to not increase debt. The first is doable, the second is not.
At debt 120% of GDP, an average interest of 4.5% plus no real growth in the next year would force Italy to run a primary surplus of about 5% next year not to increase its debt to GDP ratio. This seems unfeasible.
Until the late Great Recession, Italy had been mostly running primary surpluses. Then it was unable to do that, understandably. But now it is going to try to run a bigger primary surplus when it is already slipping into recession?
http://sdw.ecb.int/quickview.do?SERIES_KEY=121.GST.Q.IT.N.D1300.PDF.D0000.CU.EIf Italy had been a profligate spender in recent years, it is possible that it could institute the changes necessary without too deeply restraining growth. But Italy HASN'T been a profligate spender - it has been quite responsible compared to most developed countries. Therefore it doesn't have much to cut that won't have negative economic feedback.
If the US had to trim its budget by 3% of GDP next year, it would have to cut spending or raise taxes by 405 billion. In one year. And it would have to maintain that cut each year going forward. To put that into perspective, total government social spending was last reported at 2,297 billion. If you cut all those benefits evenly by 17.6%, you'd get it done. But the economy would collapse as a huge number of people took a very large income cut, unemployment would rise, and that would cut tax income and raise relative social benefit spending, and so you'd probably also have to raise tax rates by about 10% on the top third to compensate.
For Italy, it appears impossible, and it is not as if they haven't already raised taxes and cut spending, which has produced the following result:
http://www.markiteconomics.com/MarkitFiles/Pages/ViewPressRelease.aspx?ID=8785Adding US Debt as a percent of GDP chart:
The spike in interest rates back then threw the US economy into a very deep recession. But it did not make our debt servicing costs unsustainable, because our debt to GDP ratio was low and interest rates were doomed to collapse with the recession: