Investors realized last week that a resolution to Europe’s financial crisis may be far more important to the future of the U.S. financial markets than just about anything President Obama can say or the Federal Reserve can do.

A rumor Friday morning that Greece would default on its debt over the weekend sent U.S. and European shares into a tailspin, despite the Greek government’s strong denial of such talk.

Contributing to the dour mood was the resignation of Jürgen Stark, Germany’s top representative on the European Central Bank’s executive board. His departure reinforced the impression that the ECB, like the U.S. Federal Reserve, suffers from internal disagreements about how to help the economy. The discord raises questions about the effectiveness of these institutions, and more immediately, whether Germany will support the funding of future euro-zone bailouts.

Problems across the pond overshadowed the $447 billion jobs program that President Obama unveiled Thursday night before a joint session of Congress. The Dow Jones Industrial Average fell 303.68 points, or 2.7% Friday, and 248.13 points on the holiday-shortened week, to finish the four days at 10,992.13, the Dow’s first trip below 11,000 since Aug. 22. The Nasdaq lost 0.5% to 2467.99.

Signs of distress were even more apparent in markets overseas. Yields on Greek two-year notes climbed as high as 48%, and 10-year Greek paper now yields 18%. Germany’s DAX index of stocks lost 4% Friday, and Spain’s stock market fell by a similar amount.

European bank stocks were clobbered, with Deutsche Bank (ticker: D falling 8.7%, or almost three points, to 31.14. UBS (UBS) slid 6% to 11.87, and Credit Suisse Group (CS) 6.12%, or 1.49, to 22.86. Investors are uncertain about just how much exposure banks have to the sovereign debt of Greece and other ailing European countries. If the weak European nations need to restructure their debt, the banks could need to raise new equity.

U.S. financials suffered less damage, but hardly emerged unscathed. Citigroup (C) and JP Morgan Chase (JPM) shares fell roughly 4%, and Bank of America (BAC) stock lost 3%.

IF EUROPEAN BANKS ARE ABOUT to be hit by a restructuring of sovereign debt, President Obama’s stimulus plan while helpful, will not insulate American companies from the pain that’s ahead. About half the companies in the S&P 500 break out details of how much in annual revenue they derive from Europe. This group reports that 14.6% of total sales are rung up on the Continent—a fact that ought to alarm Wall Street’s most upbeat analysts.

So far, analysts are standing by their rosy earnings forecasts for 2011. S&P 500 operating earnings are expected to total a record $98.59 this year and $112.35 in 2012, says Howard Silverblatt, senior index analyst at Standard & Poor’s. But the optimism embedded in those forecasts looks to be unwarranted.

Friday brought signs that expectations might be about to come down. McDonald’s (MCD)’s August same-store sales rose 3.9% in the U.S., missing expectations for a 4.4% jump due in part to the hurricane that hit the East Coast. The chain’s sales in Europe rose 2.7%, disappointing those forecasting a 5.57% increase. McDonald’s stock lost about 4%, or 3.58 a share, falling to 85.03. Janney Capital Markets reduced its earnings estimate for this year by four cents, to $5.19 a share, and cut its ’12 forecast by five cents, to $5.65.

SEPTEMBER HISTORICALLY is a tough month for the markets, but Bespoke Investment Group notes that it has been particularly weak when the first few days of the month produce losses. The Dow’s 4.08% decline in the first three days of September was its fourth worst three-day start going back to 1900. When the market declines by 2% or more in the first three days, it has averaged a 6.31% decline for the remainder of the month.

In the three other years when the market fell by 4% or more in its first three days of September trading (1931, 1946 and 2002), the results were even worse: an average decline of 13.5% through the rest of the month. In 1946 the loss over the remainder of September was 4.83%. In 2002 it was 8.35%, and in 1931, a stunning 27%.

The results in the rest of the year aren’t pretty, either. Bespoke reports that the Dow has fallen more than 2% in the first three days of September 13 times since 1900. In those years the index fell on average by 10.37% for the remainder of the year (Source: Barrons Online).