Imbalanced combined arms

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Infantry divisions - when they still existed for real, not in name only - were known for having the infantry as their main arm of manoeuvre and combat. All other arms (usually only artillery*, sometimes also armour* and aircraft*) were supporting arms, and tactics showed this. Beginning in 1915, artillery still proved to be the most deadly arm - even if meant to fulfil a supporting role only.

Armoured divisions were known for having armour as their main arm of manoeuvre and combat. All other arms were supporting arms. Again, artillery at times exceeded the lethality of the armour, but armour at times excelled more at taking prisoners than at killing. Then, again, the infantry was supporting and especially collecting those enemy soldiers who surrendered, for armour is much better at accepting surrenders than at guarding POWs.

massed employment of armour, 1940

Artillery was rarely the main  arm. This was mostly due to the difficulty of killing much with artillery if no manoeuvre drives the enemy troops out of their positions (which were chosen in part for survivability). Some notable exceptions were sieges in which fortifications were disabled or destroyed by artillery, shelling of marshalling areas and harassing fires. Some WWI and Soviet breakthrough battles were essentially orchestrations of huge artillery concentrations, but even then artillery was primarily the arm that enabled manoeuvre by infantry (and tanks), thus fulfilling a supporting role.
Modern artillery with its new-found precision could in theory reach farther out and be most lethal even to troops in fortifications, and would in both depend on infantry (as observers) or aircraft (again, as spotters) as supporting arms. There were armoured forward observation vehicles since about 1940, but I don't recall any use of their in which artillery was the main arm for an entire battle.

17th/18th century siege in Vauban's style: Artillery breaches and suppresses,
infantry digs forward, defenders usually surrender in time to avoid the final assault.

Finally, air power. It began as a supporting arm in 1914, included substantial ground attack against front-line troops by 1917, by 1920 it was for the first time the main arm in the Somaliland campaign and by 1923 it was used without notable supporting arms in the British air terror campaign ("air policing") in Iraq. The all-air power campaign against Yugoslavia in 1999 was no true first of its kind. Air power can make use of artillery (such as ATACMS missiles expended in 1991 Gulf War) and infantry (especially observers on the ground) as supporting arms, though both not at very great depths.

It is typical of medium-sized and large modern armed forces that they strive to be able to use any of the four principal arms as the primary arm, with others meeting the supporting role. At other times I called this behaviour the pursuit of a balanced mini army (in regard to small powers).

This isn't necessarily something that they achieve and sure it isn't easy in the first place.

Armour is typically preferred in mobile warfare, but getting the use of armour right above the level of companies is difficult. Peacetime training can be done well up to platoon level, training at mixed battalion level against equal or superior opposing forces is rather rare.
To employ armour as a truly swift force, taking no more than minutes of preparations before an assault, demands much of the command system and command culture, morale, communications and training in cooperation. There's little point to armour-centric approaches in Europe if hours of preparations are needed for an advance by four kilometres in face of any resistance. Armour-centric approaches are about mobile warfare.

Several NATO armies have a reputation for being rather sluggish with armoured forces. They do not seem to have mastered the employment of armour as the main arm, though they likely are proficient in using armour in a supporting (or equal) role in tank-infantry teams.**

I had to use some pictures to break up the desert of letters.

Even the most proud NATO armies may be largely incapable of using the infantry as the main arm, due to want of quantitative infantry strength. In many brigades the ratio between armoured fighting vehicles and infantrymen is between 1:5 and 1:10, resulting in a severe shortage of infantry - and this may worsen rapidly if infantry experiences a higher casualty rate than armour faces attrition during the first days of a campaign. Infantry as the main arm may in the first weeks of a conflict thus be limited to small engagements in most tank-unfriendly terrains.

Artillery as the primary arm is quite a challenge as well. The current structure of the German army includes so very few active artillery forces that it's impossible to us (on a large scale) without a prior mobilisation and training period of months. The underwhelming ammunition stocks prohibit artillery-centric operations as well. Artillery was at times the main arm in Afghanistan, though: Infantry was moving to contact, then cowered behind cover till artillery did hit the (often only assumed) enemy positions. Those were small actions, at most up to battalion size, though - hardly battles by historical standards. A combination of 90's tech guns with 2000's fire control and munitions doesn't suffice for an artillery-centric approach that hits most targets spotted by supporting arms*** if the quantity of guns and ammunition doesn't allow for it.

Air power as the primary arm is the most expensive approach. Helicopter-centric "air mechanisation" was never more than a pipe dream or a procurement racket in Europe. Bombing campaigns - their newest version being the aerial drone assassination campaign - are not challenging against defenceless targets, but an air power-centric approach to warfare in Europe against an aggressor that felt strong enough to attack you or your alliance is most dubious. NATO, for example, would face the Russian air force with its world-beating area air defences, short range air defences and quite respected fighter force in such a scenario. Only the USAF and USN would dare to assume that they could conduct such a campaign successfully.****

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A modern first rate military should have mastered more than one of these approaches. 
Sadly, the German military seems to be a master - on scales relevant to national or collective defence - only of the armour-centric version. We're lacking in (active) artillery and infantry, and I don't think that the rather neglected Tornado ECR force (and its ammunition supply!) suffices to enable an air power-centric approach in collective defence.
The German military wouldn't fight in its "traditional" way, of course; it would not fight alone or with weak allies only. Still, there's little reason to expect allied armies to contribute the necessary artillery or infantry power to enable any other than armour-centric approaches in the first weeks of conflict.
Air power-centric approaches are dubious at best during the first weeks; wearing down the missile supply and radar inventory of opposing air defences takes a while. We're largely lacking the long range artillery munition stocks to assist air power much.

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The ability to successfully fight battles with infantry-centric, armour-centric and artillery-centric approaches could be used as a metric for an army's preparedness for national and collective defence and for its value for deterrence.

S O

defence_and_freedom@gmx.de

*: For sake of simplicity I will use "artillery" for artillery, infantry guns and mortars and "armour" for all kinds of armoured fighting vehicles, including tanks, armoured cars, assault guns and tank destroyers in this article. Same for "air power"; this includes both fixed and rotary wing aviation.

**: Which is reminiscent of the typical U.S.Army tank-infantry team of 1944/45 and of assault gun tactics. 
***: This isn't so much about "artillery conquers, infantry occupies" as it's about delaying actions with minimised exposure for duel (line of sight combat) forces.

****: The USN's Super Hornet/Growler force is kind of the definition of how much air power the United States might need for defence purposes. All additional air power is excess, necessary -if at all - only for offensive purposes.

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