Why is it that the U.S. Army, which has 40 percent fewer peacekeepers in the Balkans now than four years ago, suddenly has declared that the strain of peacekeeping is so great that two of its 10 divisions are unready for war?

At pains to untangle this riddle, senior Army officers last week gave complicated, technical explanations about staffing, troop rotations and wartime deployment plans.

All the while, they adamantly denied that the "C-4" readiness ratings--the lowest of four possible grades that division commanders assign to their units each month--were essentially a political ploy to bolster the Army's case for more money and possibly more troops.

"We weren't pressured or influenced to do anything," declared Brig. Gen. Gary Speer, assistant commander of the 10th Mountain Division, one of the low-rated units. "We were just following the rules."

But there is no question that the ratings came at an opportune time for the Army. They coincided with ongoing talks between top Pentagon civilians and Gen. Eric Shinseki, the Army chief of staff, over the prospect of extra funds in the fiscal 2001 budget proposal that President Clinton is expected to send Congress early next year.

Meeting with reporters after the classified ratings became public Wednesday, Shinseki said his service was short billions of dollars in 2001 and later years. The budget crunch, he argued, threatens to strangle the ambitious plan he announced last month to turn the Army's outdated heavy infantry into more agile combat units.

What remained puzzling was how the presence of just 12,000 Army troops in the Balkans could result in 20 percent of the service's combat force being declared unready to go to war. Pentagon officials said the problem had less to do with traditional measures of readiness--personnel, equipment and training--than with judgment calls by division commanders that their units would have trouble pulling out of the Balkans and getting to a regional war in time.

The two divisions in question--the 10th Mountain, based at Fort Drum, N.Y., and the 1st Infantry, based in Germany--are being pulled in two directions, officials said. On one hand, they have onerous peacekeeping commitments--the 10th Mountain has 6,000 troops, or about 45 percent of its force, in Bosnia, while the 1st Infantry has about the same number, or about 40 percent of its manpower, in Kosovo.

At the same time, the Pentagon's contingency plans for fighting two major wars in close succession require the divisions to be able to extract themselves quickly from peacekeeping assignments, retrain at home bases, then ship off to the second of the two wars.

Other commanders have faced similar pressures in recent years as their forces have been sent to keep the peace in Bosnia, Haiti, Somalia and elsewhere. But this time, Army officials said, several conditions have made the peacekeeping duty more problematic.

One factor is that manning levels are down in both divisions. Overall, the Army's 10 divisions fall short of authorized levels by about 6,700 troops. But Pentagon officials said the shortfall, particularly in highly specialized jobs, is even greater in the 10th Mountain and the 1st Infantry because they are designated to be among the last to deploy for war.

Moreover, when combat divisions have been given peacekeeping duties in the past, the Pentagon has designated troops to take the place of the peacekeepers in the event of war. No substitutes were designated for the 10th Mountain and 1st Infantry.

Officials also said the 10th Mountain, which did peacekeeping duty in Haiti in the mid-1990s, may be under more strain now because it has a higher percentage of its troops committed in Bosnia. And the 1st Infantry may be more challenged by the Kosovo mission than it was during earlier duty in Bosnia, because Kosovo is a harder place in which to move troops and equipment, officials said.

Even so, the officials suggested that the two divisions could regroup and go into battle faster than the ratings suggest. They promised to spell out for the division commanders some of the potential airlift and other redeployment schemes that have been left vague.

In any case, news of the Army's poor ratings clearly surprised and embarrassed the Pentagon's top civilian officials. After several years of declining indicators, Defense Secretary William S. Cohen had asserted that readiness was turning a corner as a result of increased spending and other remedies--a claim regarded skeptically by congressional Republicans, who have continued to accuse the Clinton administration of underfunding and overcommitting U.S. forces.