Is there a difference between ‘defensive’ and ‘offensive’ weapons?

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As the war in Ukraine enters its third month, many in the West support helping Ukraine to defend itself against Russian aggression. However, the United States and its NATO allies want to avoid a direct conflict with Russia that might escalate, possibly to a nuclear confrontation.

These competing goals have encouraged many Western countries to argue that they are providing only “defensive” weapons to Ukraine. In the war’s early phases, most transfers were made up of relatively short-range antitank and antiaircraft systems, ammunition for small-caliber weapons, personal equipment, and communications gear, which seemed to fit the popular intuition of such “defensive” assistance.

But as the war continues, Western military assistance increasingly includes armored vehicles, artillery, long-range surface-to-air missiles, fighter aircraft and helicopters. Some have criticized this as an unwarranted shift to offensive weapons inconsistent with Ukraine’s status as a defender, likely to provoke nuclear escalation as Russian President Vladimir Putin has threatened. Others argue that a shift to offensive weapon assistance is appropriate and necessary.

Neither position makes sense — because the distinction between “offensive” and “defensive” weapons is so blurry as to be effectively useless.

Analysts have tried to distinguish ‘offensive’ from ‘defensive’ weapons for over a century

Many scholars have long felt that some combination of range, mobility and armor protection makes some weapons uniquely suited to offensive operations — that is, to invading a neighbor rather than defending one’s borders. Tanks, for example, are often described as offensive because of their mobility and armor protection. Deep-strike missiles or aircraft are often described as offensive by virtue of their range. In an earlier era, battleships and aircraft carriers were often seen as offensive for similar reasons. This was even codified in international arms treaties such as the Washington Naval Treaty of 1922 or the Conventional Forces in Europe agreement of 1990, which limited weapon types seen as uniquely “offensive” or destabilizing in favor of others that were seen as defensive.

A long tradition in international relations theory has tried to systematize such distinctions and use them to explain variations in the incidence of war, arms races, alliance formation, and even the structure of the international system.

But these efforts suffer from several serious conceptual problems.

Most weapons can be used for both offense and defense

First, at the tactical level where individual battles are fought, almost any weapon can be used either to attack or to defend. For example, tanks can be used to attack, but they are also usually the most effective weapon to defend against enemy tanks. Tanks’ rate of fire and ammunition supply exceeds that of most infantry-carried antitank weapons, and their armor protection allows them to survive artillery barrages that would kill the exposed crews of antitank weapons such as the Javelin or the NLAW (Next Generation Light Antitank Weapon). The outnumbered Israelis who defended the Golan Heights against Syrian tanks in 1973 were using mostly tanks, not infantry, to defend.

Infantry can defend, but their small size and superior stealth enables them to exploit small terrain features to attack in ways that tanks cannot. The Taliban attackers who nearly wiped out the U.S. base at COP Keating in 2009 were infantry without tanks in support.

Long-range missiles or aircraft can strike in enemy territory, but this can help defenders by destroying the supplies attackers need or by disrupting the follow-on echelons needed to maintain an attack’s momentum. Allied attacks on Axis logistical shipping deep behind German lines helped halt Rommel’s offensive in North Africa in 1943.

Antiaircraft missiles can defend a country from an invader’s air force, but they can also protect invading ground forces from the defender’s air force. Egyptians invading Israel in 1973 used ground-based air defenses to protect their invasion force in Sinai.

Modern warfare combines different types of weapons

Second, weapons are rarely used singly or alone in modern battle. Combined arms methods have dominated since 1917. These require that ostensibly “offensive” weapons such as tanks be combined with ostensibly “defensive” weapons like infantry to succeed against skilled opponents.

Among the Russians’ greatest flaws in February was their inability to support tanks with infantry. Unsupported tank columns were vulnerable to concealed Ukrainian defenders that tank crews could not see or counter. Russia’s ostensibly “offensive” tanks needed better “defensive” infantry to attack successfully.

Self-defense requires both offense and defense

Third, any national-level military action involves both attack and defense at the tactical level. Invaders are typically attacking at a chosen point but defending elsewhere. Defenders are typically defending at the invader’s attack point, but often counterattack against the invader’s flanks, as Ukraine has done in Kharkiv and elsewhere.

Invaders with the initiative can almost always take ground early; defenders must counterattack if they are to restore what is lost. The U.S.S.R. defended against a German invasion in 1941 but required massive counteroffensives from 1942 to 1945 to retake the Soviet territory lost in 1941.

How militaries use weapons affects what they can do

Finally, any weapon’s effects are shaped powerfully by a host of nonmaterial variables, especially its users’ skills and methods.

Differences in tactics, skill and motivation swamp the military effects of most variations in weapons or equipment. The same new tank, aircraft and radio technologies that some believe created offensive blitzkrieg victories in 1939 and 1940 brought stalemates later in the war after defenders adapted. By the middle of World War II, some of the largest tank attacks in history, at Kursk in 1943 or Operation Goodwood in 1944, were turned back by defenders whose methods had changed since 1940. Equipment matters, but how it is used matters more.

Of course, changing Ukraine’s mix of weapons will affect both its offensive and defensive capabilities. But the net effect of adding tanks as opposed to antitank missiles, or aircraft as opposed to antiaircraft missiles, is complex and likely to pale in comparison to differences in how Ukrainians and Russians use such weapons. The user’s skills and methods almost always matter more than whether their weapons are ostensibly “offensive” or “defensive.”

Stephen Biddle is a professor of international and public affairs at Columbia University and an adjunct senior fellow for defense policy at the Council on Foreign Relations. His most recent book is “Nonstate Warfare: The Military Methods of Guerillas, Warlords, and Militias” (Princeton University Press, 2021).

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Stephen Biddle