Combined Arms is the strategic use of all battlefield assets in a coordinated effort to achieve objectives. Each unit has its own particular strengths which must be exploited to their fullest and its own particular faults which must either be masked or protected as much as possible. Using combined arms effectively is not difficult but mastering it takes a great deal of practice. Because combined arms is practically a science, many strategies pertaining to it are orthodox and easily understood, lacking the flair of the artist or the spark of inspiration. However, there is a reason that there is such a thing as orthodox strategy: it generally works in the circumstances it was designed for.
AIRPOWER
Air power, as shown in more modern conflicts, is a major combined arms asset. The advantages to airpower are that airpower can quickly strike at targets far behind static lines, airstrikes against ground targets are much less risky in human cost than the amount of ground forces required to do the same, and cargo airpower can move men and material very quickly from battle to battle. The disadvantages to airpower are its high costs to operate in fuel and maintenance, airpower requires some sort of organized support structure (aircraft carriers or airstrips) which limits its range, and airpower alone cannot hold ground, which is usually required to meet objectives. Airpower is best used tactically to a) destroy key points that are inaccessible and/or are extremely dangerous to ground forces and b) provide suppressive firepower, and it is best used strategically in destroying enemy infrastructure.
ARMOR
Armored units, such as tanks, tank destroyers, and armored personnel carriers (APCs), are very important in the combined arms order of battle. Their advantages are that they are highly mobile in open terrain, provide massive amounts of accurate firepower, are extremely durable, and, in the case of APCs, can carry troops on the ground much faster and in much greater safety than troops marching by foot would normally enjoy. The disadvantages to armor are that they, like airpower assets, are expensive to operate in terms of fuel and maintenance, and they suffer greatly in enclosed spaces that limit their movement due to their limited situational awareness and large size. Tanks are the kings of deserts, steppes, and plains, but are mostly large targets in cities and dense forests that limit their mobility. Unlike airpower, tanks can be operated "off the land" for short periods of time as long as fuel resources and spare parts last. Armor is best used to destroy resistance using its mobility in open terrain. In enclosed situations, armor is best used to support infantry using its heavier firepower.
ARTILLERY
The strength in artillery lies in its ability to strike at distant targets. The advantage to artillery is its long-ranged firepower, which has little to no risk to its crew; the disadvantages to artillery are its dependence upon an ammunition chain, its long distance from the target with its higher potential for friendly fire, and its relative lack of mobility. Mobility is less of an issue with self-propelled artillery guns, which can keep up with armored advances. Static artillery is best used in defensive situations; artillery in general is best used to provide suppressive fire for ground forces, directly strike at well armored or entrenched targets, or interfere with enemy supply lines if they're close enough.
INFANTRY
Infantry, the average group of human beings given light weapons and light armor and told to go out and fight, play a very important role in combined arms. Because infantry is made up of simple humans, it must be used so a human's natural ingenuity and nimbleness can be used to their fullest, and human frailty must be reduced as a factor as much as possible. The advantages to infantrymen are numerous: they have the greatest all-terrain capability of any ground force, making them useful in forests, jungles, cities, caves, and other enclosed spots; they are small, and so are far easier to entrench and conceal than armor or artillery, which allows them to set effective ambushes where they can threaten even mighty tanks and airplanes with anti-tank and anti-aircraft missile launchers; they are less expensive to outfit and maintain than mechanized modes of warfare; they are easily compatible with armor and aircraft, which can carry them quickly to and fro; they are easy to operate "off the land" as long as food and ammunition can be scrounged (and psyches hold out); and, most important of all, infantrymen are the best units for holding ground. The use of any ground held by infantry is denied to the enemy, and this is what an offensive campaign is all about. The major disadvantages to infantrymen are their vulnerability, especially in open spaces, and their relatively light firepower. Infantry are best used to hold ground and fight in close terrain.
NAVAL ASSETS
Naval assets are losing a portion of their importance due to the creation of airpower; unlike the 1600 to 1945 period, he who controls the oceans no longer really controls the world. Naval assets are now valuable in a combined-arms scheme where they can support air and ground operations, which is rare. The most notable support nowadays comes from aircraft carriers, which give airpower the supply-support it needs, and bombardment with heavy naval guns (WWII) or cruise missiles (today), which acts just like artillery. Naval assets are best used to provide long-range artillery support and resupply capability to ground forces near bodies of water; if they're inland, then naval assets are generally a non-entity.
NUCLEAR WEAPONS (same for other weapons of mass destruction)
Nuclear weapons do not have a direct role in combined
arms strategies but do have a definite impact. Since the 1950s, tactical nuclear
devices have existed to defeat armor, aircraft, naval, and infantry formations, not to
mention the thousands of strategic missiles aimed at civilian, industrial, and
infrastructure targets. However, the use of any of these weapons would seemingly
lead into a downward spiral of nuclear apocalypse defined under the concept of Mutually
Assured Destruction, which goes thusly: Two nuclear powers are at war with each
other. Both are armed well-enough to completely annihilate the other with nuclear
warheads. If either country uses even one of its warheads, lets say a tactical
anti-naval warhead, then the other country would be provoked to use bigger and more
warheads, which would eventually lead to all the nukes being fired and everybody dying.
The only way for either country to use nuclear weapons, it seems, would be to fire
them all at the same time (first-strike), which would elicit a return fire from the other
country, and everyone dies. Nuclear weapons, therefore, keep conflicts from getting too
hot because of the ominous spectre of nuclear winter. That is why the USA and the
USSR never had it out--it wasn't in anyone's best interest. What is far more
dangerous are nuclear powers with only limited numbers of warheads that won't annihilate
everybody--India and Pakistan, for example. The US dropped its bombs on Japan
because the US had them and Japan didn't--no threat of MAD.
But I digress. The advantage to nuclear weapons in a combined
arms sense are their threatening stature and their ability to serve out copious amounts of
distruction. Their disadvantages are as numerous as the advantages of infantry:
they completely wreck territory, making it unusable; they can easily lead to a
world-wide nuclear conflagration; their long-term fallout effects are a chaotic factor;
they are often too big to get a specific job done; they nullify any political
objective of a campaign by reducing the moral stance of the user; et cetera. Nuclear
weapons are best used, if at all, to scare the enemy into not using theirs.