Fugen-s in Gefechtsstärke

Question

I'm wondering why you use an "s" to join "Gefecht" and "Stärke" in Gefechtsstärke as opposed to using no interfix? Is there an acceptable rule according to which this spelling can be explained? Is there, perhaps, an etymological reason? According to my (perhaps, not the best) language feeling, I'd join them without any letter at all, though I don't have a supporting argument for the spelling Gefechtstärke either. References are welcome.

If I see it right, neither http://www.spiegel.de/kultur/zwiebelfisch/zwiebelfisch-der-gebrauch-des-fugen-s-im-ueberblick-a-293195.html nor https://de.wikipedia.org/wiki/Fugenlaut mention such general rules. Still, we could ask for general patterns for, say, compounds of the form X[s]Y, where X is a neuter noun of the form Ge...cht, and Y is a female noun starting with st. I tend to think that if X[s]Y for such X and Y is semantically the same as "Y des Xs", then you use the interfix "s", otherwise you don't. (Counter)examples are welcome, as well as better patterns.

Answer 1

The simple rule is: There is no rule.

The Fugen-s in compositions of substantives has, apparently, developed from a genitive form of the composition (this seems to be the reason why an "-s" is used):

• des Gefechts Stärke --> Gefechtsstärke
• der Realität Verlust --> Realitätsverlust
• des Schnellzugs Wagen --> Schnellzugswagen (Swiss German), but Schnellzugwagen in Germany
• die Erklärung der Einkommensteuer --> Einkommensteuererklärung
• der Hochzeit Kleid --> Hochzeitskleid!!

As you can see from the 3rd and 4th examples, even this most obvious apparent "rule" is already broken by regional developments or administrative language. The fifth example is one that inserts an -s, even if this cannot have been caused by the genitive.

For your specific example Gefechtsstärke, Google finds a lot of (mainly) historical references that don't use the "Fugen-s", especially in official announcements of government and military. So, apparently, Gefechtstärke used to be a partially accepted form for some time, at least in the military.

The Fugen-s is inserted intuitively by native speakers according to the syllables that collide at the "Fuge" to make pronouncement a bit easier - for some combinations, an added "s" seems to be mandatory, for others, not (Zwiebelfisch and some other sources come up with a few examples that can be considered hints, but not rules). Even those combination hints are often not followed by common practice and with regional differences. Intuitive usage doesn't necessarily apply rules ...)

The German Rechtschreibrat has not bothered yet to come up with a definite rule (or, you could argue they evaded the topic because they couldn't find any rule). Apparently, you need to learn all those cases.

Answering to a comment: One of the assumed reasons for the Fugen-s is easier pronunciation and understanding. Compound words can come up with combinations of consonants that don't normally appear in German, like "td" (Institut[s]direktor), "tst" (Erbschaftssteuer) and would be hard to pronounce or combinations that are hard to hear, like "mm", "tk", "nnb"("Interimsmanager", "Hochzeitskleid", "Mannsbild").

Answer 2

»Is there an acceptable rule according to which this spelling can be explained?« - No, there is no such rule. As a consequence, there are regional different usages of Fugenlaute:

In Germany (except parts of Bavaria): Schweinebraten (e)
everywhere else: Schweinsbraten (s)

Austria: Adventkalender (-)
everywhere else: Adventskalender (s)

Rheinland: Speisenkarte (n)
everywhere else: Speisekarte (-)

Switzerland: Jahrzahl (-)
everywhere else: Jahreszahl (es)

As another consequence, there can be more than one variation in different words:

Körperschaftsteuer (-)
Körperschaftsstatus (s)

Ertragsteuer (-)
Ertragssteigerung (s)

Bahnhofshalle (s)
Hofhund (-)

Even if the second part is the same, there can be both forms in use:

Namensforschung (s)
Namenforschung (-)

Erbschaftssteuer (s)
Erbschaftsteuer (-)

And sometime the presence or absence of a Fugenlaut can change the meaning of the word:

Zugführer = leader of a platoon (aprox. 50 soldiers) (in DE and CH)
Zugsführer = corporal (in AT) (»Oberstabsgefreiter« in DE)

Also note these differences:

Manneskraft
Männerhose

Also note, that those Fugenlaute often seem to represent a plural or a genitive form of the first word, but this is not true. Some experts think, that such grammatical flexions might be the etymological root, but if there was a plural or genitive meaning in the past, it is lost today. Fugenlaute are a category different from any other grammatical category.