If Saddam Hussein has in fact maintained stocks of prohibited chemical or biological weapons, he might be hard-pressed to use them against invading troops, experts said yesterday.

That is because the delivery systems he employed so successfully in the 1980s during Iraq's eight-year war with Iran are not likely to work as well today. In that war, Iraq showered Iranian troops with thousands of bombs, rockets and shells carrying chemical agents, killing or injuring more than 20,000 people, according to an October 2002 CIA report. The use of chemical weapons turned the tide of war and eventually led to Iran's decision to accept a truce negotiated by the United Nations.

"The problem for Iraq today is that the U.S. will control the air," said Jonathan B. Tucker, director of the chemical and biological nonproliferation project at the Center for Nonproliferation Studies and a former U.N. weapons inspector. "And the massed hundreds of pieces of artillery needed to deliver tons of chemical agents to be effective on a target would be an easy target itself."

Tucker said Hussein "may very well use whatever he has" in a "last-ditch defense situation." He added, however: "He could contaminate areas, but against the U.S., even if used effectively, it can only slow the oncoming forces down -- since [American soldiers], unlike the Iranians, have protective suits."

The extent of Iraq's chemical and biological weapons capability today is still unknown; but what can be understood is how Hussein chose to use -- or not to use -- his once-vast stocks of chemical agents in the past.

Iraq hastily developed its chemical weapons program in the 1980s, when it found itself losing the war it had started against the revolutionary regime of Ayatollah Ruhollah Khomeini. There were early missteps: Tucker recalled that Iraq fired mustard gas shells from valleys at Iranian troops on higher ground until "it found the heavier mustard gas coming back down on its own troops."

As the war progressed, however, the Iraqis developed sophisticated tactics that used the characteristics of different agents to their advantage. In one battle, for example, highly persistent mustard agents were used to force Iranian troops into a narrow path. Iraq then attacked with nearly 1,000 shells loaded with nerve agents, which created a covering cloud that killed more than 1,000 Iranian soldiers.

When the Reagan administration learned of Iraq's use of chemical weapons, the United States not only failed to protest but also kept supplying arms and intelligence to Baghdad. Also during those years, private U.S. labs supplied some strains of biological materials to Iraq. Only after chemical agents were used against the Kurds in the town of Halabja in 1988 did Secretary of State George P. Shultz raise a protest.

When Hussein seized Kuwait in 1990 and the first Bush administration put together a coalition to oust the Iraqis, there was great concern among the U.S.-led coalition forces that the Iraqi leader would use the massive stocks of chemical weapons to protect his victory. But Hussein made different decisions against this new, more sophisticated enemy.

In a Pentagon paper circulated in 1990, the Joint Staff noted factors for and against Hussein using chemical weapons in two situations. One was if he tried to advance from Kuwait into Saudi Arabia; the other was if the United States counterattacked and moved from Kuwait into Iraq.

The Joint Staff noted that if the Americans sought to attack from Kuwait, the prevailing winds would favor an Iraqi chemical attack because they usually blow eastward toward any incoming U.S. troops. Also, the Pentagon group said the "low humidity of the air makes it capable of accepting and dispersing more [chemical] agent." Another factor cited then and now are high temperatures, which can have an impact on the effectiveness of soldiers who must don heavy and cumbersome protective gear.

The Joint Staff noted, however, that "Hussein would be reluctant to use chemical weapons if it meant poisoning his own lands and water" as he sought to halt a U.S. invasion.

The most "likely Iraqi employment tactic" for chemical weapons would be "to neutralize a convoy column or possibly to deny the use of certain areas." The Pentagon paper said that Iraq would probably not use persistent mustard gas in an invasion of Saudi Arabia because that would create problems for its advancing forces. "It is possible, however, that the Iraqis might target rear-area installations and populations to instill fear and induce surrender."

However, just before the Persian Gulf War started in 1991, then-Secretary of State James A. Baker III delivered a letter from the president to the Iraqi government that threatened retaliation if chemical or biological weapons were used. Although the Pentagon later said that none apparently were used, it was later disclosed that Hussein had turned some chemical weapons over to commanders for use should the coalition forces enter Iraq and cross a line 200 miles south of Baghdad.

In testimony before Congress, Charles A. Duelfer, former deputy executive chairman of the U.N. Special Commission on Iraq, said he had discussions with high-ranking Iraqi officers who confirmed the Baker-delivered threat deterred Iraq from using chemical weapons in 1991. They also said chemical weapons would have been used if U.S. forces had kept going to Baghdad and that they believed that knowledge had somehow deterred the United States from overthrowing Hussein at that time.

In a 1997 paper, Tucker said that the "remarkable speed of the coalition advance, combined with the effectiveness of the strategic bombing campaign," had made it difficult for Iraqi commanders to select targets for any chemical attack. In addition, he wrote, the prevailing winds for six months in the 1990-1991 period had "blown from the northwest out of Iraq and shifted at the beginning of the ground war to the southeast towards the Iraqi lines." Thus, had they used such weapons, the result could have been threatening to Iraqi troops.

When the war ended, U.N. inspectors supervised destruction of "more than 40,000 chemical munitions, nearly 500,000 liters of chemical agents, 1.8 million liters of chemical precursors, and seven different types of delivery systems including warheads for ballistic missiles," the CIA study said.

A fully equipped Marine relaxes in a dugout in northern Kuwait near the border with Iraq as U.S. forces began their move northward into position.Wearing gas masks and chemical suits, members of the 15th Marine Expeditionary Unit huddle in a trench during a drill in the northern desert of Kuwait over the weekend.