(We will refer the reader to a YouTube Video demonstrating the basic tactic.)
Why is this a problem? One word: mobility. They are slow, and presumably tire quickly if forced to march too far with their pikes prepared.
What to do? Well, imagine a hammer and an anvil. The hammer (weighing only a fraction of the heavy anvil) makes quick strikes against heated metal resting against the anvil.
Why not do likewise? The Macedonian Phalangites form the "anvil", the enemy's infantry form the hot metal. What's our "hammer"?
Cavalry! Preferably heavy cavalry, capable of kicking ass. Recall the Battle of Leuctra, where the cavalry fought each other in a side show.
Why not use the cavalry to attack the rear? This would impact moral the most, causing the enemy to flee or die trying.
Lets stop and ask ourselves: What is this cavalry like, exactly? The Cambridge Ancient History (vol. 6) observes:
It was around this mass of semi-heavy infantry, set in the centre of the line of battle, that the remainder of the troops formed. On the right it was usually flanked by some 1,800 Companions (hetairoi) recruited from the Macedonian nobility; these were horsemen in helmets and corselets, armed with lances and swords and always ready to charge, in wedge formation, into the slightest breach to rout the enemy. On the left were posted 3,000 or 4,000 more lightly armed horsemen, some of them of Macedonian origin (scouts, prodromoi or pikemen), but mainly drawn from allies, such as Greeks, Thessalians, Thracians and other Balkan peoples, while on the extremity of the flank and at other points as required by the course of events were various units of light infantry, armed with javelins, bows and slings. Many of these were also recruited in Macedonia and in the Balkan countries, for example Illyrians, Thracians and Agrianians, but they also included such mercenaries as had not been allocated to garrison service. As for the 7,000 Greek hoplites who, within the framework of the Confederacy of Corinth, accompanied Alexander into Asia, there is little mention of them in the accounts of the campaign before their wholesale conversion into mercenaries.(Cambridge Ancient History, vol. 6, pg 687)
Note that, although we did not investigate any battle, the tactic discussed does not need one (according to Clausewitz, at least). We are discussing a tactic abstractly, so it suffices to refer the reader to a YouTube Video.
Defending Against Hammer and Anvil?
How to defend against the "Hammer-and-Anvil"?
The first thing to note is the cavalry need to pound the flanks and/or rear. If this is denied to them, there is hope to break the "anvil".
We should recall that the Infantry Square has neither flank nor rear. So one option is: form a giant rectangle.
This naive implementation has severe limitations (marching in formation would be incredibly difficult to pull-off in practice).
On the other hand, having extra units on the flanks and a reserve which can (at a moment's notice) fight off the cavalry "hammer" would be a wiser approach.
The square is no longer a unit formation, but a loose battle-field tactic. This might sound far-fetched, but Thucydides notes the Athenians (and their allies) at the First Battle of Syracuse took a similar formation:
The next day the Athenians and their allies prepared for battle, their dispositions being as follows: Their right wing was occupied by the Argives and Mantineans, the centre by the Athenians, and the rest of the field by the other allies. Half their army was drawn up eight deep in advance, half close to their tents in a hollow square, formed also eight deep, which had orders to look out and be ready to go to the support of the troops hardest pressed. The camp followers were placed inside this reserve. (Thucydides Histories, Book VI, §67)
Note the Athenians' (and friends') formation resembled the standard fighting line, with the reserve forming a hollow infantry square.
I must confess that the details of this battle are unclear to me, but the Athenians won a close victory...then quickly sailed back to their base at Catana.
Fuller reports that (at the First Battle of Syracuse): "The battle opened with a skirmish between the archers and slingers, under cover of which the heavy infantry closed in on each other. The Argives drove in the enemy's left; the Athenians penetrated his centre, and the Syracusans were only saved from annihilation by their cavalry slowing down the Athenian pursuit" (Military History of the World, vol. I, pg 65).
But also observe: we have thus far discussed how to deal with the hammer. What about dismantling the anvil?
In general, the rule of thumb is: spearmen/pikemen are vulnerable to swordsmen or shock infantry.
So if the center of the battle line is swordsmen, the flanks are spearmen (in case the companion cavalry charge the flanks), and a reserve of pikemen (again, in case the companion cavalry attempt to charge the rear)...then both the hammer and anvil are threatened.
Are these the only ways to counter the Macedonian hammer and anvil?
No! We can recall the flaws in Phalanx formations, namely, missile troops can cause serious problems.
So modifying the infantry square tactic to have a "burrito structure": the pikemen tortilla wrapped around the missile filling.
Next time, we will begin investigating some battles of Alexander the Great.