Back in the days of mechanical weapons like swords and axes, the approaches to combat differed probably even more than today. This was certainly so when it comes to clashes between small groups of men.
One approach - example mid Republican to early Imperial Roman (heavy) infantry - featured a huge shield as a most visible equipment. The scutum varied in shape over the centuries (early shape; see my top banner), but it was always a shield which permitted its user to cover himself almost entirely with it. This was actually quite possible with round shields (even the Germanic kind) as well, but the scutum was practically made for it.
This shield in combination with a decent helmet (the Roman legionary helmets became very sophisticated) allowed the infantryman to survive even ferocious attacks quite well.
A legionary with such a shield did not need to emphasise offensive action; he could wait till there was an opportunity for stabbing the enemy with his sword or till another legionary stabbed the same in a flanking attack. Opportunities were used, but it wasn't necessary to actively create opportunities to win.
Warriors using two-handed weapons and still lacking the sophisticated and superb armour of late medieval times were very different. Long axes (once popular especially in Northern Europe) or romphaia two-hand sickle swords (Thracia; ~ ancient Romania/Bulgaria) enabled and forced a different style of combat. Users of such weapons would hack at their enemy's shield or helmet, split these defensive means and thus create opportunities (if especially the hit on the head wasn't decisive in itself).
OK, so far a nice analogy for other cases of waiting for opportunities / forcing the creating of opportunities, but forget about fitting the phalanx into his framework - it doesn't work. The phalanx was probably about winning without needing much of an opportunity.
The traditional division of combat is between offensive action and defensive (in)action. I suppose another perspective for looking at it is a division between opportunity-creating and opportunity-awaiting.
The result isn't the same separation of actions as in offensive / defensive.
Delaying actions which involve ambushes would fall into the defensive and opportunity-creating categories. Sure, the ambushers wait for the hostiles, but the act of setting up an ambush is creating the opportunity of engaging hostiles in a killing zone. On the other side, some attacks don't fit nicely into the opportunity-creating category, and especially so if the opportunity already existed prior to the attack (reducing the attack to the equivalent of stabbing an temporarily exposed belly with the gladius).
Here's another way of dividing actions; attrition and 'maneuver'. Looked at from the 'opportunities' lens, attritionists don't place much emphasis on exploiting opportunities (event hough they likely produce them), resembling the phalanx exception above. 'Maneuvrists' on the other hand may fall more into the "opportunity-creating" category. There are exceptions, though; the "maneuver school"-associated "recon pull" approach (earlier French version; "manoeuvre à posteriori") is as little opportunity-creating as it gets with scouting: Recon pull is meant to find opportunities, but at the same time not necessarily meant to provoke or produce one. It's the equivalent of the legionary raising his head above the shield in order to see the gap in his opponent's defence. That's quite a different analogy than splitting some shield to hit the man behind it with a follow-up blow.
The opportunities lens may also be helpful to address the troubles around the concept of (gaining/having) "initiative" (related blog post). I suppose most of the time when someone speaks or writes about gaining the initiative, he's probably thinking of creating opportunities.
It is no perfect fit at all, but it may be more instructive and promising to order a subordinate to 'create and exploit opportunities' than to tell him to 'gain the initiative'. The latter appears to be excessively correlated with purposeless activism.
There may be some potential for insights in pushing one's awareness for the opportunity thing. The creation of opportunities comes either at a cost (risk, for example) or it's a no-brainer. The trade-off done by a leader when (s?)he decides to create (provoke, force) opportunities should probably not be done unconsciously, but consciously. Unlike "defensive" and "offensive", the "opportunity" lens offers a way of sorting options in a way that actually provides some usefulness in itself. Defensive and offensive is largely about terrain in a land warfare context, and fighting for terrain has built up a bad reputation since 1915. The opportunity lens is more directly, and at the same time more generally, about overcoming hostiles' resistance in an efficient manner.
The real challenge is of course to exploit opportunities when they arise, and it doesn't matter whether you were creating or awaiting it once there is a worthwhile opportunity.
Some military bureaucracies pride themselves on their abilities, including (though in other words) their ability to create opportunities. Yet at the same time, the very same organisations usually cannot pride themselves on their rate of actually exploiting opportunities.
Again, pushing the opportunity thing more and thus making people more aware of its relevance may be helpful.