If the United States had to fight a war tomorrow, it could not hope to win without its ''citizen soldiers.'' One-third of the marines in all-out combat would be reservists. Nearly 60 percent of all tactical airlift would be provided by ''weekend warriors'' of the Air Force Reserve and National Guard. Two-thirds of all Army combat service support would come from reserve and guard units. More than three-quarters of all mine sweepers would be drawn from the Naval Reserve Fleet.
Reserve components, says one analyst, ''are inextricably linked to the national security of our nation.''
The Pentagon - on its own and with some firm nudging from Congress - is taking steps to improve reserve and guard forces. And there is no doubt these forces are noticeably better than at their low point of the late 1970s, when the number of reservists - with the threat of conscription no longer hanging over them and official attention focused elsewhere - fell by more than 50 percent.
Yet, some experts are warning that in these post-draft days, the reserves are the either the ''stepchild'' or the ''Achilles' heel'' of the all-volunteer force.
According to a new Congressional Research Service report, the Army Reserve has on hand just 34 percent of its wartime requirement for equipment. The report warns that in some reserve specialties that would be crucial to battlefield success (particularly doctors and other medical personnel), the reserves ''are significantly short of personnel.''
''You have many units running around in jeeps pretending they're tanks,'' says Warren Lenhart, author of the congressional report. ''It's almost unbelievable that you have Army Reserve units that have the responsibility for repairing the M-1 (tank), but they've never turned a bolt on one of the things.''
''Major equipment shortfalls exist in many areas of the Surface Reserve,'' the US Naval Reserve director, Rear Adm. Robert F. Dunn, told a House Armed Services subcommittee earlier this year. Some naval air reserve units are flying obsolescent aircraft that could not be properly serviced or repaired aboard ship.
With all the recent talk about beefing up US and allied conventional forces to prevent nuclear escalation in time of conflict, the issue of being able to respond quickly is becoming very hot. And the trend here is toward relying more and more on reserves to beef up active-duty units quickly to wartime levels if necessary, and to fill many crucial support jobs that would be essential in the first days of mobilization.
''If war breaks out, some reserve units are going (into action) before many active units,'' Mr. Lenhart says.
This worries some analysts. ''There has been a longstanding controversy about how many of the reservists actually would report when ordered to active duty upon mobilization,'' John Brinkerhoff, a retired Army colonel and former deputy assistant secretary of Defense, said recently.
Senior officers and Pentagon civilians point to improvements in guard and reserve forces.
Under President Reagan, the Pentagon has earmarked more money for the procurement of up-to-date equipment for all seven reserve components (Army, Air Force, Navy, Marine, and Coast Guard Reserves, and the Army and Air National Guard).
Lawrence Korb, assistant secretary of defense for manpower, reported recently that the number of those assigned to reserve units now is near the all-time high set in 1959. And the number who are part of the Individual Ready Reserve (primarily active-duty veterans with time remaining on their total service obligation, but not assigned to specific reserve units), although less than one-third of what it was 10 years ago, is coming up as well.
''Today, we are in better shape than we have been at any time in the recent past,'' says Major Gen. Stephen G. Olmstead, Marine Corps deputy chief of staff for reserve affairs. But on Capitol Hill - where many lawmakers are veterans and the reserve and guard popular - this is not enough.
Sen. John W. Warner (R) of Virginia (who is a former secretary of the Navy) says, ''Our reserve components are a real defense bargain. They can successfully perform their missions with only 20 to 50 percent of the funds required for similar active-duty units.'' While some dispute these figures, pay and benefits costs are less for part-time military people.
Senator Warner has led the effort to cap active-duty military personnel levels and shift some missions and personnel to the guard and reserves. Congress this year authorized lower active-duty levels and more reserve manpower than were sought by the administration.
Lawmakers also directed that more money be spent on weapons and support equipment for the reserves, authorized the President to name a new assistant secretary of defense with full-time focus on reserve affairs, and directed the Pentagon to look for more ways to shift military missions to reserve units.
Congress told the President he can extend a recruit's total military obligation to eight years. While this would not extend active-duty enlistments, it would (after several years) create a larger pool of veterans for the Individual Ready Reserve. Those who stay in the IRR will be offered reenlistment bonuses as well.
But some experts warn against expanding the role of citizen soldiers too rapidly. ''Before you talk about expanding missions,'' says congressional researcher Warren Lenhart, ''you really have to decide whether or not they're capable of doing the mission they're programmed to do right now.''