Combined Arms is the use of different kinds of combat arms in concert; typically this is the combined use of some mix of infantry, artillery, armor (tanks and other armored fighting vehicles or AFVs) and airpower (when analyzing much older armies, we often talk about combined arms as mixing shock infantry, missile infantry and cavalry, so the “arms” being combined vary from era to era). This is important because effective use of these arms in modern warfare requires them to be used together: artillery and aircraft need infantry to help identify targets to strike (since both engage targets they typically cannot observe), while infantry needs them to provide supporting fires. AFVs of all kinds (but especially tanks) typically have poor situational awareness (there’s only so much you can see through even a good set of optics while buttoned up in a tank) and at close range are vulnerable to infantry-carried anti-tank weapons and so require infantry to screen them and be their eyes (while the superior firepower of AFVs enhance the infantry). Note that this can mean that good systems (e.g. an effective tank) can still be ineffective if not used appropriately with supporting arms (e.g. screening infantry with supporting fires). In the opening days of the invasion [of Ukraine in 2022], Russia struggled to use these arms in concert effectively, with the result that the Russian advantage in vehicles and fires, which was considerable, wasn’t realized. That in turn makes it difficult at this point to make clear assessments about the effectiveness of Russian equipment or the overall usefulness of certain kinds of equipment (e.g. tanks) when used properly in a combined arms framework.
Bret Devereaux, “Miscellanea: A Very Short Glossary of Military Terminology”, A Collection of Unmitigated Pedantry, 2022-03-25.
