What do we really know about this most elusive tactical manoeuvre? I spent a lot of time researching this key issue, and would like to present a sum up of what I read in this two-parts blog post. Part 1 is about comparing the two main interpretations of diekplous, and why they are both not fully convincing (to me): part 2 will be about a specific theory, which I fully buy.I hope this whole thing will help you clear your own head and find your favorite interpretation of diekplous, and have a clear idea of what your little crews are (probably failing to) doing on the gaming table.
First of all, there is a fairly large consensus among academics on what the diekplous is. It is, writes J.F. Lazenby, "the passing of a ship, or ships, through and out of an enemy line of battle", with the goal of then getting into a position "to attack the vulnerable sides and sterns of enemy warships". That is exactly what the red trieres here below is doing: it exploits a gap in the stationary enemy line (1), then turns around (2) and finally rams the stern of an enemy ship with the fury of an Erynis (3).
We also know that the diekplous was not aimed at shattering enemy oars, even if this could happen as an accidental outcome. This idea got some traction in early studies of ancient naval warfare, but it is today almost completely repudiated, so we can take this off the table.
Finally, we know that the diekplous, during or around the Peloponnesian war, was one of the two manoeuvres performed by expert crews and/or faster ships (the other being the periplous). This is why I chose undecked triremes (ALKEDO 5 and 6 at Irregular Miniatures' shop) against fully decked units to represent the diekplous in the pictures. Landlubbers or heavy, slow ships, Thucydides explained us, would limit themselves to just rowing towards the enemy, board them with marines and beat the crap out of them by hand - what lack of finesse!
The crux of the matter is, how did the diekplous fit into a wider tactical picture? How was it performed during naval battles? There are mainly two schools of thought here. The first I will call the "line abreast school". According to this school of thought, diekplous was a manoeuvre executed individually by single ships, starting from squadrons drawn up in "normal" formation (i.e., line abreast). You can see an example in the picture below, in which the captain of the red trieres (again) performs a diekplous, starting from its own line of battle.
The clearest proponent of the “line abreast" theory is good professor Lazenby himself, but the theory has many supporters, including apparently also the venerable W.L. Rodgers. Lazenby's main argument is philological in nature. He argues that ancient sources overwhelmingly indicates that ancient fleets fought in line-abreast, and that the only two instances where a line-ahead diekplous supposedly took place, Lade and Arginusae, never actually happened. It was merely a mistranslation, they were not actually in line-ahead. My ancient Greek is only slightly better than my Sumerian, so I will not discuss this argument in more detail, but the whole thing seems to revolve about the expression “epi mias": if it is used to mean “in single file" (i.e., line ahead) or “one (ship) deep" (i.e., line abreast). To sum up, Lazenby and the "line abreast" school argue that no ancient source ever told us that a fleet engaged another in line ahead, and that therefore diekplous must have been performed in line abreast.
I kind of agree with this theory, on one hand. I can totally see how one single ship could and would perform a diekplous if it should spot a gap in the enemy line. And, indeed, I find it hard to believe it could be a squadron tactic: this would imply that there should be a number of wide gaps all along the enemy line, to allow several ships to pass through at the same time. But, on the other hand, if the diekplous was only a single ship tactic consisting in exploiting an hole in the enemy line, I doubt very much it would get a specific name and so many mentions as a specific battle tactic by ancient Greek historians. Why would historians tells us that squadrons would get into position "ready to do both the diekplous and the periplous", if diekplous was just a smart move by one ship? After all, we don't have a specific name for the movement of an hoplite enlarging a gap in the enemy phalanx by hitting exposed flanks of enemy hoplites, do we? Because this would be the obvious and natural thing to do for an hoplite, that you do not need to have a name for it - they would just do it as the most evident option.
The second school, the “line ahead" school, postulates that the diekplous was executed by whole squadrons, and that these squadrons would be formed in line ahead. In the picture below, the red trieres is not only performing itself the diekplous, but is leading its squadron in exploiting of the gap in the enemy line. The following ships would pass through and, after turning, choose their targets.
This view is supported by no less than J. Morrison, the builder of the replica trieres Olympia. Always on the basis of philological analysis of ancient writers, he argues that the “modern" maneuvers for fast triereis would be the periplous and the diekplous, both of which performed by squadrons launched at sustained speed and in line ahead against the enemy. This way they would, in his view, have a better chance of getting through the enemy line, albeit with some losses: while the same maneuver by single ships would be “self evidently suicidal", as he writes.
Morrison also explicitly responded to Lazenby on the “epi mias” issue. Again, I could build a pentekontor by myself more easily than I could debate ancient Greek philology, so I will merely report that, according to Morrison, ancient sources indeed tell us that diekplous was used (at Lade, Salamis and Arginusae for example), and they also tell us that it was performed by whole squadrons deployed in line ahead. Actually, Morrison argues that ancient writers took for granted that the normal formation for a 15 ships squadron planning a diekplous was 3 files of 5 ranks. Therefore, diekplous could and would be used by squadrons in line ahead. Granted, says Morrison, the breakthrough was dangerous to the leading ship, and that is why commanders would resort to it when there was no sea room for a periplous (this, he says, was what happened at Salamis).
Common sense tells me it would indeed be quite dangerous to the leading ship and the other ships, because of the exposed flanks. And indeed Lazenby comments Morrison's theory as totally impossible. The lead ship of the line, he says, would have an "almost suicidal" task, and if it – or any ship behind it – was to be disabled or rammed, all the succeeding ships would have their way blocked. Moreover, ships attempting to diekplous in a column would be terribly at risk to be rammed on the side or stern.
In conclusion, I personally did not find either school particularly persuasive. The "line abreast" theory, in my view, was not convincing as it seemed to minimize the nature of diekplous as a specific maneuver, reducing it to a smart move by an aggressive captain. The "line ahead" explanation, on the other hand, seemed to me to be impractical to actually perform on the field, for the reasons stated above. I decided I needed a detailed discussion of what ship movements/tactics would be realistically achievable, on the basis of what we know by archeological sources, and how exactly would they achieve those movements.
Luckily, this is exactly what was done by Andrew Taylor in an article titled “Battle manouvers for fast trireme". It is actually a chapter in the Olympias final report. It demonstrates, with a dynamic mathematical model based on the Olympia's performance, how the diekplous could practically be performed. Taylor shows the different ships’ movements step by step, in different situations. His work is so cool, and his conclusions so persuasive, that I want to discuss it in proper detail, so this will be the subject of the second part of this post.
I was requested a few references on the sources I used for this article, so there you go.
For the "line abreast" school, the key read would be J. F. Lazenby's article "The diekplous", published in Greece & Rome, vol. 34 issue 2 1987. Others are, for example, A.J. Holladay ("Further thoughts on trireme tactics", Greece & Rome vol. 35 issue 2 1988) and the radical Morrison critic H.T. Wallinga ("The trireme and history", in Mnemosyne vol. 43, fasc. 1/2, 1990), but also, ambiguously, the old admiral W.L. Rodgers ("Greek and Roman naval warfare").
On the "line ahead" school the key players here is J.T. Morrison, both in his seminal book "The Athenian trireme" (a must-have, I reviewed it here), in the chapter "The trireme" in Conway's "Age of the Galley: Mediterranean oared vessels since pre-classical times" (another must-have, I need to review it still) and in the very interesting article "The Greek ships at Salamis and the diekplous", in Journal of Hellenic studies, vol. 111, 1991. But he's not alone: there are, for example, the already mentioned Andrew Taylor in his chapter in "Trireme Olympias: the final report", ed. by Boris Rankow, or J. Myres in his article about "The battle of Lade, 494 BC", in Greece & Rome vol. 1 n. 2, 1954.