Two men knock on the door of a family whose son is fighting in the Persian Gulf. The strangers give the impression of being emissaries from the military, and they are solemn. "Your son has been arrested for possession of marijuana in Saudi Arabia. The courts are rough over there. We might be able to get him off, but it will cost about $5,000."

It's "Operation Desert Con" by the opportunists who scurry out of the woodwork to capitalize on wartime fears. The drug-bust hoax is one of the bolder cons reported at a recent internal briefing of Army officials in Washington.Families of servicemen and women are easy marks. They may not know exactly where their loved ones are. Letters and phone calls are spotty. The machinations of the Pentagon are a mystery. Families learn more from CNN than they do from official sources and letters. So when someone appears at the door with "news," the family may act first and ask questions later.

Consumer watchdogs and military officials told our associate, Jim Lynch, about a variety of recurring cons. Families have been phoned by someone who tells them that their soldier has returned to the United States injured. "Please come and pick him up." When the family rushes out, the robbers sneak in.

A few 900-number bandits have crawled into the act. They leave messages on a home answering machine assuring the family that their soldier is alive and well. "If you want more information, call this number" - a 900 prefix toll call.

Along with the scams are the legitimate, if opportunistic, businesses making a buck off of patriotism. Overnight markets have cropped up for "Cookies for Combat," "Official Desert Storm Dog Tags" and even "Desert Shield Cologne."

The New York attorney general investigated Voices For Freedom because of consumer complaints that the Virginia company overbilled credit cards when callers ordered Desert Shield bracelets for $9.95. Even first lady Barbara Bush has one of them. The company now says the billing "glitches" were the mistake of telephone solicitors and that the problem has been taken care of. The company says its profits paid for a voice-mail center where soldiers could leave three-minute messages to be sent to their families.

Another enterprise that has taken considerable heat is "Help Hospitalized Veterans," which sells "G.I. Gift Packs" for $15 and gives part of the proceeds to charity. The boxes reportedly contain about $5 worth of goodies. The company president, Roger Chapin, says it would cost the average citizen $19 to assemble and mail the same package. The last time we wrote about Chapin, two years ago, he was running an outfit called "Citizens for a Drug Free America," with a direct-mail campaign to raise money for a citizens lobby to fight the drug war.

Ken Albrecht, president of the National Charities Information Bureau, a charities watchdog, claims that historically, less than 60 percent of the donations to Chapin's umbrella organization, "Help Hospitalized Veterans," actually go to bona fide charitable services. Albrecht warns potential donors to ask plenty of questions before handing over their money, and he says the best way to help soldiers is through the Red Cross or USO. Even if the package service is a bargain, it may not be the best way to bolster the troops. Military sources tell us that delivering gift packages - homemade or mass-produced - is a low priority after vital supplies. Gift packs may sit in U.S. warehouses for some time.