“How much defense is enough defense?” is the great unanswerable question of defense economics. Those charged with preparing a defense capability tend to be more cautious about the level of capability than those who eventually have to pay for it. In fact, the very success of deterrence—a high probability of nonattack throughout a long period of peace—tends to reduce the amount of defense spending that the electorate considers necessary to achieve deterrence. Judging the appropriate level of military preparedness is not a science; it is a mixture of intelligent response to credible threats and of judicious, cautious preparation “just in case” this or that should arise. The managers of the armed forces tend to increase the contingencies they wish to prepare for, while skeptical taxpayers tend to question whether certain preparations are absolutely essential. In democracies this tension forms the permanent agenda of the defense debate.

Defense expenditures are made on an annual basis, the government allocating so much of its total budget to personnel costs, so much to the procurement of weapon systems, and so much to general support. The pay and allowances of defense personnel are consumed within the year; that is, they spend their wages, allowances, and pensions on consumer goods and, in so doing, add to total demand in the economy. Procurement, on the other hand, is somewhat different. A tank lasts much longer than the single year in which it is purchased. Because it is supposed to last as long as it takes to become obsolescent, the tank becomes part of the country’s permanent defense capability. That defense capability is, in economic terms, a stock, while the annual expenditure is called a flow.

Even if, for some reason, a defense budget is reduced in a single year, a country’s defense capability need not be reduced. The government can still draw on the stock paid for by previous defense budgets, which is manifested in its tanks, aircraft, ships, communications systems, trained personnel, and expertise in military affairs. Clearly, if the defense budget continues to be reduced every year, there will come a point at which the country’s defense capability will decline through attrition as items of equipment become obsolete or beyond repair.

The analogy is with a bath that is filling with water while the plughole is open. As water pours into the bath, water also drains from the plughole. It is the difference between the rates at which water flows in and out that determines whether the bath fills or empties. If the flows in and out are equal, the water level will remain constant. Likewise with defense capability: if the additions (flow in) to the stock of weapons matches the attrition (flow out) of the stock from all causes, then the country’s defense capability will remain constant.

Budgeting a nation’s defense capability is complicated, however, because defense capability is not determined unilaterally; it depends on the capability of the potential aggressor. The gap in military capability between any two countries is known as the threat, and estimates of the threat constitute the major input into defense planning.

If a potential aggressor develops an advanced weapon system that effectively counters a weapon stocked by the defender, it will eventually threaten to overwhelm the latter’s defenses. Likewise, a growth in the stock of weapons deployed by a potential aggressor can eventually have a similar effect in threatening to overwhelm the defender’s smaller stocks. If the defending country does not invest in overcoming each new threat to its capability—by technology, new types of weapons, increasing the stock of current weapons, or all three options simultaneously—it will risk a reduction in the probability of nonattack—that is, its deterrence capability will be compromised.

Estimates of the threat of a Soviet invasion across the German border determined the nature of NATO’s response for more than 40 years. While NATO planners considered their own forces to be technologically superior to the Soviet forces, they were nevertheless mindful that the Soviet Union had a decisive quantitative superiority in conventional forces (more tanks, armoured vehicles, artillery, combat aircraft, and troops). The threat of a land-based invasion by Soviet forces, which the planners considered to be virtually unstoppable, led directly to the decision to deploy nuclear weapons as the ultimate deterrent against an invasion of western Europe.

Nobody could survive a major nuclear war in Europe. The damage to the Soviet Union from an American nuclear strike would be matched only by the damage to the United States from a Soviet nuclear strike. Because each country has maintained sufficient nuclear forces to respond in kind to a first strike by the other, a nuclear exchange would be suicidal for both. Whatever the rhetoric, therefore, both countries have a strong interest in preventing war of any kind from breaking out on the continent of Europe. Literally, they are hostage to each other’s behaviour, making Europe an unsafe place to start a war. This doctrine, known as “mutual assured destruction,” was given the appropriate acronym MAD.

The consequences of MAD led NATO to adopt a policy known as “flexible response.” Rather than an all-or-nothing nuclear exchange, this envisaged a staged escalation of NATO’s response to a Soviet invasion, based on containing the initial thrust of the Soviet forces and warning them of the consequences of further encroachment on NATO’s territory. To underline the credibility of the threat of nuclear retaliation, NATO commanders were issued battlefield nuclear weapons, which NATO governments might or might not release for immediate use, with or without warning. Uncertainty about NATO’s policy of probable first use of nuclear weapons was regarded as sufficient to make Europe an unsafe place for the Soviet Union to risk the consequences of a conventional war. As long as the risk of the horrendous consequences of a nuclear war exceeded the prospects of potential gain from launching an attack, the probability of nonattack on western Europe by the Soviet Union remained at an acceptable level.

The possession of nuclear weapons by some NATO countries (the United States, Britain, and France) did not obviate the need for expenditure on conventional armed forces. To abandon conventional forces would risk having to use nuclear weapons as soon as the first Soviet forces crossed the German border or some naval incident occurred in any part of the world. This escalation from a small incident to the end of the world in one short step was unacceptable; hence, NATO countries invested resources in conventional capabilities in addition to nuclear weapons. These conventional forces aim to blunt a Soviet attack and give time for political processes to influence the Soviet government’s decisions.

Matching conventional forces to Soviet conventional capabilities had to take note of two facts: First, the Soviet Union had overwhelming superiority in conventional forces. Military doctrine holds that concentrating superior force at a single point can overwhelm the defense, and the Soviet Union had the capability to achieve such a strategic advantage at a time and place of its choosing. Second, while NATO had advantages in military technology, there was a constant effort by the Soviet Union to close the technological gap. Also, there is some point at which a quantitative advantage acquires a qualitative dimension, and this advantage cannot be neutralized solely by relying on a technological gap between the weapon systems themselves.

Thus the paradox of NATO defense spending. The alliance was constantly trying to widen the technological gap to compensate for its disadvantage in numbers, while at the same time it was required to maintain large quantities of its existing systems to redress the ever-widening gap in numbers that the Soviet Union was believed to be creating across the German border. Whether to develop ever-new weapon systems to combat a closing of the technological gap by the Soviet Union as well as the sheer numbers of Soviet systems, or to concentrate on supplying the armed forces with duplicate copies of existing designs, has long been NATO’s quandary.