Soon after the financial system collapsed four years ago, a new phrase entered the lexicon of banking: too big to fail.

There may now be an equally infuriating corollary: too big to punish.

London-based HSBC laundered money for drug lords, including Mexico's notorious Sinaloa cartel. The bank and its subsidiaries also helped customers in Libya, Iran and other pariah nations skirt financial sanctions, according to federal prosecutors and their counterparts in New York.

In a four-count complaint filed last week in U.S. District Court, prosecutors accuse HSBC of violating the federal Bank Secrecy and Trading With the Enemy acts. The bank lacked an effective anti-laundering program and failed to do basic due diligence on clients, according to the complaint.

Yet the U.S. Justice Department opted not to pursue HSBC or its executives as long as they behave for five years. Why? According to a New York Times report, some officials feared that a criminal case might "jeopardize one of the world's largest banks and ultimately destabilize the global financial system."

In other words, HSBC is too big to punish. Or at least too big to indict.

ING Bank, JPMorgan Chase, Wachovia and Citigroup also have been cited for money laundering or sanctions violations over the past two years. None of those cases resulted in criminal charges either.

This week, however, the Justice Department filed criminal charges against a major bank and two of its employees — the first such case since 1989.

A subsidiary of the Swiss bank USB agreed to plead guilty to one count of wire fraud in an interest-rate manipulation case. A second bank, Barclays, already paid a fine but faced no criminal charges.

Among the victims in this case are U.S. mortgage giants Freddie Mac and Fannie Mae, which may have lost $3 billion due to rate rigging schemes.

Prosecutors said the criminal charges don't threaten the financial system because UBS is shielded from losing its charter. Was that not an option in any of the other cases?

That question hasn't been answered, not now nor when the HSBC case was settled. At that time, prosecutors emphasized the $1.9 billion in penalties to be paid by the bank. Assistant Attorney General Lanny Breuer called the settlement a "very just, very real and very powerful result."

Maybe so, but it wasn't very painful.

For HSBC, the sanctions amounted to a little more than 8 percent of annual profits for 2011. But bank officials said they were sorry — make that <i>very</i> sorry — and the bank's shares went up 0.56 percent on the day the settlement was announced.

Stiff financial penalties might be a catalyst for stockholders to demand better behavior from the managers of big financial institutions.

For now, however, sanctions, rather than being a deterrent, seem to be another business cost for those institutions deemed too big to fail. It may take more prosecutions to get their attention.