As HE CAMPAIGNS, President Reagan sometimes suggests that the Carter administration simply let the country's defense deteriorate until the Reagan administration took over. "What we inherited when we came here," he said in a speech last week, "was an America that over the years had unilaterally disarmed." The historical record tells a different story.

If you look up the actual figures for American defense spending, adjusted for inflation, you will see that they dropped sharply in the early 1970s. That's no surprise. The country was coming out of the Vietnam War, and those were also the years of Soviet-American d,etente. But by the middle of the decade a good many influential Democrats, as well as the Republicans who were then in power, began to think that the reductions had gone too far. They noted that there had been no corresponding slowdown in the Soviet Union. The long decline in defense spending ended in the spring of 1976, under President Ford. Spending remained constant through several budgets and then, in President Carter's last two years in office, rose steadily and purposefully. That trend has been greatly accelerated by Mr. Reagan, who can justly claim to have increased the emphasis on defense much faster than Mr. Carter did. But the rising trend was established in the late 1970s, not in the 1980s.

One important question is whether defense spending is always an accurate indicator of defense strength. While it was Mr. Carter who began the presng, it was also Mr. Carter who firmly implanted the present custom of using the annual rate of increase as a signal to friends and adversaries of American intentions in security policy. Mr. Reagan has embraced this practice as well, and expanded it. The trouble with it is that, as a political convention, it draws attention to the amounts being spent and away from the ways in which they are being spent. That does not necessarily lead to greater care or efficiency in defense spending.