By most measures of war-fighting capability, the United States armed forces today are better off than they were when the Reagan administration took office. Whether the uniformed military is adequately prepared to defend the homeland and protect US interests abroad is a highly subjective issue involving political perceptions, shifting international relations, and arms control considerations. And any judgment here must note that the renewed emphasis in national defense began during the latter years of the Carter administration.
But there are many indicators that the US military is in better shape to fight today than it was three years ago:
* Personnel factors - recruiting, retention, educational level of those in uniform - are markedly better. This is largely because of substantial pay raises , reenlistment bonuses, slower duty rotation, and efforts at better unit cohesion. This is true of high-skill officer categories - pilots and nuclear submariners, for example - as well as senior enlisted personnel.
* The composition of ground forces is moving slowly but steadily toward more mobility and the ability to react more quickly, as military reformers say it should. This includes the development of light-infantry divisions and Special Forces units trained and equipped to deal with the kinds of ''low-intensity conflict'' most likely to occur.
* Force modernization - the replacement of 1960s-'70s weaponry with new generations of equipment - is well under way. The Army had just 34 of its newest tanks (the M-1 Abrams) in 1980; between 1981 and 1984, another 2,929 were funded. And many bugs are being worked out. One hears much less criticism now about the performance of such controversial items as the M-1, Aegis cruiser, F- 18 fighter, and Pershing II missile.
* The services have developed and now are are using computer models and other high-tech equipment (including lasers) to simulate combat without having to fire increasingly expensive live ammunition. Service schools have also become better able to pattern the Soviet equipment and battlefield doctrine they most would likely face.
* Production and stockpiling of combat consumables - ammunition and spare parts - have lagged behind funding for ''big ticket'' weapons, but there have been clear increases here as well. These are now beginning to fill armories following the typical two-year production cycle. And proposed budgets for the rest of the decade show a tailing off in new procurement and a catch-up in this ''sustain-ability'' funding, which is projected to double during the 1980s.
''The bottom line, simply stated, is that while we're not yet where we'd all like to be in terms of war-fighting ability, we are considerably better off than we were several years ago,'' the Marine Corps commandant, Gen. Paul X. Kelly, declared recently. ''And the important point to remember is, we're getting even better.''
Much of the recent debate over military readiness has to do with how the services measure war-fighting capability and also with the definition of two major parts of defense funding: procurement and ''O&M'' (operations and maintenance).
The Army and Air Force have shown some decreases in their units' measurement of combat readiness. But senior officers insist that this has to do primarily with tougher measurement standards recently adopted by the services. Beginning last year, for example, certain critical aircraft spare parts were given greater weight by the Air Force.
Officers also note that units slated to get new equipment - the M-1 tank, for example - receive a lower rating even though their combat capability with the older weapons hasn't actually dropped.
Critics say the Reagan administration has overloaded the defense budget with money for new weapons at the expense of funds to operate and maintain them.
In a recent study, Rep. Les Aspin (D) of Wisconsin, a member of the House Armed Services Committee and frequent Pentagon gadfly, took issue with this analysis and reported that the Defense Department in fact may be overfunding its operations and maintenance account. He found that much of the money designated for procurement is actually used for support purposes, including service life extension programs, ammunition, support vehicles, and replenishment spares.
When this more accurate division of procurement and O&M is used, Representative Aspin showed, operations and maintenance during the latter half of the decade will grow at twice the rate of new weapons procurement, thus making up for the ''front-loading'' of Pentagon spending with new weapons during the first Reagan term of office.
There is no quarrel among defense analysts, though, about the improvements in military personnel since 1980. The number of new recruits with high school diplomas has jumped from 68 percent of the total to 92 percent (better than the overall average of American youth). The portion of enlisted personnel with four or more years of service has increased nearly 20 percent. And the number scoring in the lowest acceptable test category dropped from 31 percent to 8 percent.
Some defense analysts say high unemployment in earlier years sent young men scurrying to military recruiters. According to military personnel officials, however, recruiting has been just as high in communities with relatively low unemployment.
Senior officers are quick to say that they have not reached their goals, especially in levels of weaponry, spare parts, and ammunition. But as they look at the forces they command today, most agree with General Kelly of the Marines that there has been ''significant improvement.''