Where US defense is weak

America is absent-minded about its own ethnicity. I think it is about the only country that is this way. When my grandfather sat in the New Hampshire legislature after the Civil War it was just taken for granted that the American racial stock would continue to be predominantly Anglo-Saxon. The tide of variegated immigrants was only starting to rise. Now the population pattern has changed drastically. Other great rivers have flowed into the central pool. Even to raise the ethnic issue is often regarded as racist.

I know of no other country where conditions are parallel to this in scale and consequence. The United States will defend its borders, of course. President Reagan will ask Congress this year for a military budget of $245 billion or so. That protects the United States militarily. But to protect the physical borders the government assigns only about $77 million for its border patrol. This is grossly inadequate. There are only about 350 patrolmen on duty at any one time. I do not believe that there is another such example in the world. Take the 2,000 -mile Mexican border, for example. It is porous. Nobody knows for sure, of course, but estimates are that the number of illegal Mexican aliens in the United States about equals the 12 million US workers unemployed.

Former Immigration Commissioner Leonard Chapman estimated in July 1976 that only one of every three or four persons attempting to enter illegally was apprehended.

The casualness of this is what startles the foreigner. Nearly everybody agrees that the situation needs tightening. A high-level presidential commission studied the matter. It produced the compromise Immigration Reform and Control Act (Simpson/Mazzoli) in 1982. This proposed sanctions on employers who knowingly hired illegal aliens, and methods of identification. The Senate passed the measure overwhelmingly, 80 to 19. Did it pass the House? Not at all. It never got a vote. Now the work starts over again. On any dark night on the Mexican border bands of illegals enter. As broadcaster David Brinkley said on his show Nov. 14, ''Illegal immigration is now almost totally out of control.'' There is wide agreement.

This situation, of course, affects America's economy. It involves job supply, workers' wages, and the simple question of whether the nation can enforce its own policies. The attraction of employment here is a magnet that brings illegal entry from around the world. A federal law forbidding employers to hire aliens lacking work authorization cards would dull or remove the magnet. The proposed bill has compassionate provisions recognizing the plight of those illegal aliens who have lived here a long time. But now we are back at space one, the starting point.

The United States has brought its own population almost under control; natural increase is about 0.8 percent a year. Mexico's natural increase is three times that, around 2.5 percent a year, one of the highest in the world. Mexico is almost broke. The surge to come to the United States, legally or illegally, has rarely been higher. It is true of many other hard-up countries, too. Confronted by this global situation the United States can't make up its mind to take the stern steps of restraint to preserve its relatively advantageous position. We waver.

On Dec. 8 the Mexican Senate (according to Federation for American Immigration Reform, the local interest group which watches such things) unanimously passed a resolution questioning the US policy of controlling immigration and expressing ''our alarm and concern for the repercussions which will impact both countries if the Simpson/Mazzoli legislation is passed.'' The Mexican statement continued that ''this transcendant matter should not be considered from a unilateral perspective, but rather should be treated from a bilateral and even multilateral perspective, taking into account the far-reaching migratory phenomenon of undocumented persons between our two countries.'' The Mexican Senate referred proposed stricter US immigration policies to the Latin American Congress, the World Congress, the Group of Parliamentarians for a New World Order, and to its own Foreign Relations Committee.

Hi-ho. The Mexican Senate says it's not our business; the US Congress says nothing and US unemployment hits 10.8 percent.