The financial crisis in Europe seems very complex, but we understand that how it comes out will have important and perhaps painful consequences for Americans as well as Europeans. At its center is the fear that if Greece defaults on its debts that could endanger the health of European banks, and that in turn may cause a financial crisis not unlike what followed the collapse of Lehman Brothers in 2008. How did we get into this fix, so soon after 2008?

Last week, EU leaders agreed on a rescue plan for Greece that involves investors (primarily banks) writing off 50% of the value of their loans. Is this another case of the banks doing something dumb, or is there more to the story of these investments in Greece?

As a guide for the perplexed, here are some Qs and As that might shed some light on why we are where we are:

What's the underlying cause of this crisis? High debt-to-GDP ratios among the Europe's southern tier countries, resulting in an increase in the risk--and a decline in the value--of their outstanding debt.

What's the effect? The banks--primarily European--that hold this debt have been seriously weakened by the reduced value of these assets. If Greece actually defaults, the debt could become almost worthless.

Why did the banks buy this debt? Bank regulators from around the world encouraged it.

What? How did they do that? The current bank capital rules (known as the Basel rules after the Swiss city in which the regulators meet) give banks a strong incentive to hold sovereign debt.

What kind of incentive? The Basel capital rules make sovereign debt cheaper for banks to hold than other kinds of debt.

"What's the underlying cause of this crisis? High debt-to-GDP ratios among the Europe's southern tier countries, resulting in an increase in the risk--and a decline in the value--of their outstanding debt."--Peter Wallison

Can you give me an example? Sure. Bank capital is basically equity, common shares or their equivalent. It's the first to suffer losses so it's very risky and thus very expensive.

So? Under the Basel rules, banks must allocate at least 8% of their capital to support their loans to corporations, and less than half that for the mortgages they hold. It's called risk-weighting of assets.

OK. How much capital must they hold against sovereign debt? None

You mean the debt of all European countries has a risk weighting of zero? Yes

Even Greece? Yes

Why would the Basel rules treat the debt of all governments the same? Because the rules are made by bank regulators from around the world, all of which are agencies of governments. Governments like banks to buy their debt.

Could it be that the Basel rules would not have been adopted if the debt of all the participating governments had not been given the same zero risk-weighting? Yes

Does this mean that the Basel rules may have caused the financial crisis in Europe? Yes

Isn't this a severe indictment of the Basel rules? Of course.

What would we do without these rules? The market would decide which government's debt represents zero risk.

What's wrong with that? Nothing

Then why were the Basel rules developed in the first place? Regulators were worried that governments might decide to lower the capital standards of the banks chartered in their countries, giving them advantages against banks in other countries and making them riskier.

What were they afraid of? A race to the bottom. Basel is an attempt to assure standardized capital requirements for all internationally active banks.

But didn't governments, through zero risk-weighting of sovereign debt, just give themselves the advantages they were afraid might be given to the banks? Yes

And isn't that the cause of this impending crisis? Yes

So how do we get out of this? Ask your favorite bank regulator.

Peter J. Wallison is the Arthur F. Burns Fellow in Financial Policy Studies at AEI