In World War II the United States was, in Franklin Roosevelt's memorable phrase, the ''arsenal of democracy,'' the industrial and technological giant whose miracles of production ensured the ultimate defeat of the German and Japanese war machines.

Today there is growing concern among military strategists over the U.S. ability to perform a similar role if the situation demands. They're afraid the increasing number of defense contracts awarded to foreign companies will erode the industrial base that enabled this country to turn out planes, tanks and other materiel at a rate never seen before or since.

In fact, the Pentagon already depends on foreign suppliers for many vital parts. Ninety percent of all military ball bearings smaller than 30 millimeters are imported, as are 80 percent of the armed forces' semiconductors. Even 2 percent of the precision components for U.S. guided missiles are supplied by foreign firms, while almost all the military's steel nuts and bolts come from foreign manufacturers.

We are becoming increasingly dependent on foreign sources for critical raw materials, which could conceivably be cut off in wartime. Vital military materials that are mostly imported include asbestos, chromium, manganese, mercury, mica, nickel, potassium, titanium and zinc.

At first glance, the amount of defense contracting by foreign companies doesn't seem all that dangerous: almost $9 billion out of $136 billion in 1986. But that compares with $5.4 billion in defense contracts awarded to foreign bidders in 1980. And the problem of U.S. industrial readiness is compounded by the corresponding drop in U.S. military sales to other countries -- from $14.8 billion in 1980 to $7.1 billion last year. The plant capacity represented by arms exports is important because it could be transformed to production for U.S. armed forces in the event of war.

Retired Gen. Richard Thompson, former commander of the Army Materiel Command, shares the opinion that the United States lacks an adequate industrial base for defense purposes. He points out that because modern weapons are so sophisticated, it would take a long time to convert peacetime plants to military needs. Thompson is also concerned with the movement of the U.S. economy from manufacturing to service-oriented industries, which have little or no military potential.

A production expert in the Pentagon, John Mittino, has described another facet of the problem, writing: ''Many key industries, essential to expanded production in a national emergency, have encountered serious problems as a result of global competition. These problems of . . . diminished technological and manufacturing capability have not been limited to smokestack industry, but have struck high-technology industry as well, with semiconductors being a notable example.''

What can be done? Rep. Dick Durbin (D-Ill.) came up with one answer.

Until last year, tank wheels were among the many military spare parts -- from computer chips to fiber optics -- that were open to bids from foreign companies on an unlimited basis. Durbin successfully lobbied the Pentagon, with the result that now at least half of the Army's tank wheels must be supplied by U.S. manufacturers.

The congressman's concern over tank wheels was not coincidental, by any means. A company in Quincy, Ill., in his district, manufactures the wheels used on heavy machines. With the drastic cutback in production of tractors and other farm machinery, the company, Can-Am, saw great possibilities in a switch to tank wheels.

A company official told our reporter Gary Clouser that since the regulation requiring at least half the tank wheels be bought from U.S. firms, the company has been awarded five Pentagon contracts totaling $20 million. Can-Am had been consistently underbid by an Israeli competitor on tank-wheel contracts, and under the old procurement policy, the Israeli firm got the contracts.

Countering objections that the new requirement for tank wheels is a form of protectionism, Durbin argued that the need to maintain defense readiness is more important than concerns about retaliation over limits on the number of, say, videocassette recorders that can be imported.

Durbin, a third-term representative, had the help of Illinois' two Democratic senators, Alan Dixon and Paul Simon, in his effort to change the Pentagon's policy. Encouraged by his success on tank wheels, Durbin is now trying to have the Pentagon -- and the State Department -- agree to require that at least half of all contracts for vital defense items be reserved for domestic manufacturers.