Updated 7/7/2017 to improve readability.

Since the end of the Cold War, many states have opted to remove certain munitions from their arsenals. Sometimes, states choose to decommission munitions because their operation and sustainment is costly. In other cases, munitions fall out of favor for humanitarian reasons. Cluster munitions, some of the most destructive conventional weapons currently in use, fit into the latter category.

What are cluster munitions?

Cluster munitions were invented in the early 20th century as a way to increase the area damaged by a single bomb. Normal (unitary) bombs and warheads contain one large explosive charge. Cluster munitions, on the other hand, contain a number of small submunitions which are dispersed in mid-flight (see image). Each of these mini-bombs, or bomblets, has its own detonator and explosive charge. When the cluster munition opens, each submunition’s path is variably affected by the wind and other factors, resulting in the submunitions dispersing over a relatively wide area.

Many cluster bomblets inflict damage through blast fragmentation. In other words, each submunition simply detonates like a small bomb. These types of cluster munition are often used against large targets such as surface-to-air missile (SAM) sites and military bases. Because of submunition dispersion, one cluster bomb or warhead can eliminate a large SAM site that would have required multiple unitary munitions to destroy.

Because each submunition is relatively small, the individual bomblets do not have a lot of explosive power. For this reason, fragmentation cluster bombs are primarily useful against soft (unarmored) targets. Other types of cluster bombs, such as the CBU-100 Rockeye II, use explosively-formed penetrators (EFPs), allowing them to defeat armored targets.

While anti-tank and blast fragmentation cluster munitions are the most common, a wide variety of payloads can be packed into a bomblet. Some cluster munitions contain incendiary submunitions, while others are used for minelaying or even pamphlet distribution.

What makes cluster munitions so dangerous for noncombatants?

The main drawback of cluster munitions is their potential to produce unexploded ordinance (EXO). EXO is any explosive munition that fails to detonate during its deployment. To illustrate the EXO danger posed by cluster munitions, let us consider a hypothetical situation in which bomblet fuses have a 5% failure rate (which is on the low end of what many EXO experts would cite).

Many cluster bombs carry over 100 bomblets; assuming three cluster bombs are dropped with 150 bomblets each, there would be 450 bomblets total. Given the 5% failure rate, those three cluster bombs will have produced around 22 unexploded bomblets.

This simple hypothetical illustrates the danger of cluster munitions: the massive number of bomblets essentially guarantees that EXO will be produced. Striking the same target with conventional bombs, on the other hand, is unlikely to produce EXO as there are only a few fuses that could fail.

Compounding this issue is the small size of cluster munition EXO. If a 250lb bomb fails to explode, it is relatively easy to identify and disarm. Cluster munitions, on the other hand, are quite small. Thus, they can easily go overlooked for years or even decades when they fall into brush or become covered in dirt. Small submunition size combined with the potentially large number of unexploded bomblets make cluster munitions highly dangerous for noncombatants and costly to remove; a light jostle could be all it takes to detonate a years-old bomblet hiding in the grass. Cluster bomb submunitions can also be mistaken for harmless toys or innocuous scraps of metal, especially by children. In many cases, submunitions can even be defused and have their explosives harvested by insurgents or terrorist groups for use in improvised explosive devices.

Because of these dangers, a multitude of nations have signed or ratified the Convention on Cluster Munitions, which bans all signatory states from producing and employing cluster munitions. Others, however, refuse to sign. Many of the states which have not signed the convention possess powerful militaries (for example, Russia, China, and the United States). Others are in areas of geopolitical conflict — few Middle-Eastern or Asian states have ratified the convention.

Many countries, such as the US, keep cluster munitions but use them sparingly. While the US maintains a stock of cluster munitions for use in high-intensity warfare, it avoids using them in most cases; precision munitions such as guided missiles and bombs are preferred.

Other nations, such as Russia, are less discriminating. In Syria, where the US has mostly employed precision munitions, Russia has used many unguided bombs, reportedly including unguided cluster bombs.

In the end, each individual nation has to make tough choices regarding cluster munitions. On one hand, pressure from NGOs and disarmament groups certainly influences militaries to dispose of their cluster munitions in order to avoid the associated noncombatant casualties and negative publicity. On the other hand, cluster munitions can be valuable in certain tactical situations, especially for the elimination of large area targets including forward bases, staging areas, infantry formations, airfields, SAM sites, and artillery batteries. Surrendering the cluster munition capability could equate to a loss of combat effectiveness, and many militaries (such as the US) have stopped using cluster munitions regularly, which allows them to save the capability should it be needed while avoiding the EXO casualties of regular employment.

Another solution is to field more advanced cluster munitions, such as the CBU-97 Sensor Fused Weapon, which contains only 10 submunitions. Each submunition dispenses four smaller units, which acquire targets individually and then fire kinetic penetrators. This design reduces unexploded ordinance rates and collateral damage significantly, but advanced cluster bombs such as the CBU-97 are very pricey. Weapons such as the CBU-97 are part of America’s goal to reduce submunition dud rates to below 1%.

Thus far, the trend has been for African and European states to join the Convention on Cluster Munitions while most of Asia remains non-signatory. The Americas are a more mixed bag; many states have ratified the treaty, but heavyweights Brazil and the US have not yet followed suit. Of course, the disarmament debate remains lively, and if/when the dust settles in Syria and Iraq, new insights into the impact of cluster munitions could inform future decisions. However, there are no real indications that Russia and the US will swear off cluster munitions anytime soon. Seeing as these countries drop the majority of ordinance and have the largest stocks, any efforts to end the deployment of cluster munitions will need a breakthrough with these two countries.