Washington — Three years ago, the Army's chief of staff, Gen. Edward C. (Shy) Meyer warned Congress that he was presiding over ''a hollow Army.'' Four of 10 US-based divisions were not combat-ready; supply and ammunition stores were low; experienced personnel were leaving and there were too few recruits.
Things have changed a lot since then, and now the caissons are rolling along with a renewed vigor that would make any old beans-and-bullets soldier proud.
The Army ''is attempting the most ambitious force buildup'' of any service branch, according to one expert. It's dramatically reorganizing its forces and fielding a new generation of weapons and other hardware. Recruiters are having to turn away more applicants than they sign up, many of those accepted have to wait months before induction, and those who are accepted reflect the best peacetime quality ever.
After years of post-Vietnam decline, the Special Operations Forces (Green Berets) are being rejuvenated. With the Pershing II intermediate-range missile and renewed interest in antiballistic systems, the Army also is flexing more nuclear muscle. The chairmanship of the Joint Chiefs of Staff has shifted from the Air Force to the Army.
''The Army is embarked on one of the most ambitious transformations it has ever attempted in peacetime,'' General Meyer told the House Armed Services Committee this week.
Most defense analysts agree that Army manpower, weapons, and battlefield doctrine indeed are undergoing significant change, much of it along the lines urged by military reformers.
The Army's changing posture is designed to support the new NATO shift to counterattacks behind enemy lines and second-echelon strikes against attacking forces in Central Europe. Maneuver warfare is replacing the old doctrine of firepower and attrition.
But all of this is not without its critics. Several of the new major weapons (M-1 tank, Bradley infantry fighting vehicle, DIVAD air defense gun, Apache helicopter) have suffered performance problems and cost overruns.
Some say the new gear will require unacceptably long supply and maintenance chains, that the trend will actually decrease combat flexibility, and that high-technology weapons could be found wanting in the chaos of battle.
There is no doubt, however, that ''this is an Army in transition,'' as General Meyer said this week. The fact that the Army has begun insisting on better physical fitness among its 780,000 soldiers - linking it to promotion and special schools, removing those from service who don't comply - has more than symbolic significance.
Army officials point to what they call ''force multipliers'' as key to providing more capability from US land forces. Among these:
* Shifting strategy from static defense to more aggressiveness and mobility. Called ''AirLand Battle,'' this includes anticipating enemy movements and disrupting his advance by attacking reinforcing elements. ''In doing so, we gain battlefield leverage we have not heretofore possessed,'' says General Meyer.
* Concentrating Army resources on new equipment instead of more soldiers.
This decision, made two years ago, is now seeing fruition in a new array of Army weapons. Many of these are highly sophisticated, relying on advanced sensors to detect enemy movements, and ''smart'' weapons designed to have extreme accuracy at long range.
* Increasing unit stability and cohesion and thus combat effectiveness.
The Army is beginning to form its operational groups at the training stage and keep them together for several years of deployment. Unit commanders also are being rotated less often. In test units for what the Army calls its COHORT project, retention is up, discipline problems are down, there are fewer racial and ethnic cliques, and greater unit pride is evident, officers report.
* Making the Army less reliant on the Navy and Air Force for getting its troops and materiel to trouble spots.
Leaner, lighter, more mobile units are being formed at Fort Lewis in Washington. Similarly, the Green Berets (which dropped from 13,000 men to 3,600 after Vietnam) will grow to 5,000, reflecting the administration's heightened concern over terrorism, subversion, and other threats calling for the talents of those schooled in ''unconventional warfare.''
Not everyone agrees that all of this will work as advertised or that it is necessarily a good thing, however.
In a recent report on the Reagan administration's defense buildup, the relatively conservative Heritage Foundation took a detailed look at the Army's restructuring effort and concluded that: ''In general, the movement is toward fighting organizations that are larger, heavier, and more dependent for direction on high level, centralized points of command and control.''
''The division will grow much heavier in logistics,'' wrote analyst George Kuhn. ''[The] intelligence gathering system will rely on a complex - and vulnerable - network of communications and monitoring equipment covering the entire combat theater. As a consequence of greater reliance on a centralized command network, much command initiative will shift to rear areas. . . . The chaos of battle will sorely test these cumbersome arrangements.''
Mr. Kuhn warns that AirLand Battle ''is reminiscent of many failed attempts to impose a grid of certainty on warfare.'' Weapons like the M-1 tank require much more fuel and more frequent maintenance, he says, and history shows that ''ground conflict will be won or lost in close encounters,'' not with high-technology long-range systems.
While military reformers in Congress and elsewhere have been urging a shift to maneuver warfare as outlined in the new Army plans, they too are suspicious of overly complex weapons that could fail - or be irrelevant - if they are ever truly tested.