When is a promise not a promise? People who loaned money to subprime homebuyers have one view. Those who bought stock in 1st Mariner Bank at $10 a share have another. (It's now $1.) So do those who expected long careers at Black & Decker and were laid off in a brutal recession.

In an age of diminished resources, bad faith and dashed hopes, everybody's feeling jilted.

But few groups sound as aggrieved as the government employees whose pensions are being cut by financially stressed states, cities and counties.

"The City Council and mayor have turned their backs on police and firefighters," Robert Cherry Jr., president of the local Fraternal Order of Police union, warned The Baltimore Sun two weeks ago. "They should expect police and firefighters to turn their backs on them."

Last week, the City Council cut future contributions to fire and police pension funds by hundreds of millions of dollars to avert a budget crisis. The minimum service time before retirement was raised from 20 years to 25. The formula for cost-of-living increases was revised. Employees must contribute more to the fund, and so forth.

Fire and police unions sued the city even before the reform bill passed. Cherry says they're talking about seeking an injunction to block the changes.

"It's just disappointing that we had to move forward with the federal suit," he said in an interview last week. "That is our plan. Our members are demanding it. We believe that the city made a promise and has broken that promise."

It's a thumbnail image of what's going on across the country, as state and local governments deal with huge budget shortfalls and pension benefits that many claim are too generous. It's also a preview of what Maryland faces next year, when whoever wins the governorship will be forced to deal with a roughly $30 billion abyss in funds that finance pensions and retiree health care.

Traditional, "defined benefit" pensions are one of the riskiest financial commitments anybody can make — or receive. Workers joining pension systems in their 20s are promised explicit payoffs that won't come for another half a century. In a topsy-turvy world, such money is far from guaranteed, no matter what the employee-benefits brochure says.

And for government employees, the promises are substantial.

Unions note that average annual pensions for government retirees are relatively low, often in the $10,000 or $20,000 range. But these calculations include many retirees with only a few years of service, which drags down the average. Baltimore firefighters and police who retired in fiscal 2009 with more than 30 years of service are getting average pensions of almost $63,000 a year, according to the funds' annual report.

(Note: Fire and police employees don't get Social Security. And it goes without saying that they also have dangerous jobs in a dangerous city.)

Teachers, state agency employees and other members of the Maryland State Retirement and Pension system who retired in fiscal 2009 with at least 30 years of service are getting $35,000 a year on average.

Corporate employers can adjust these kinds of commitments fairly easily. When a union contract expires, they renegotiate the accumulation of future benefits for current employees. Or they change terms unilaterally for nonunion folks. Or they go bankrupt, which can hurt retirees already drawing a pension.

It's a lot harder for governments. They're run by politicians who get co-opted or pressured by unions, which are far more numerous for public than private employees. Baltimore's fire and police locals are already saying they'll try to eject council members who voted for the pension haircut.

What's more, a long time ago some Baltimore pols granted extraordinary legal protection to city employees. The city's charter — its constitution, essentially — says that each new member of its pension systems "shall be deemed to have entered into a contract with the Mayor and the City Council." Once the employee joins a city system, the charter says, pension benefits cannot be "diminished or impaired."

Holy accrued actuarial liability, Batman.

The unions' lawsuit will be closely watched because other states and cities have the same kind of language in their existential documents. But saying that a contract exists does not make it inviolable.

"There is some case law that says you can impair contracts for a significant public purpose," says James E. Spiotto, a Chicago lawyer who has studied the issue closely and followed Illinois' severe pension problems. "If it's a choice between providing essential government services and funding the pension — that may happen."

Years of accumulated hazards now threaten government pensions. True, Baltimore, Maryland and other governments underfunded pensions for a long time. But pensions also face dangers — slow economic growth, a bum stock market and retirees living a lot longer — that few foresaw.

Expect the Baltimore lawsuit to be aggressively litigated but settled out of court. The unions will give up much, although maybe not as much as City Council took away last week. Teachers and other state employees can expect their own haircut next year.

With the economy likely to be weak for years and public employees getting retiree benefits that have all but vanished in the private sector, the political and financial resources to maintain them just don't exist.

No matter what the fine print says, a government pension, like all politics, is the art of the possible.