There is a great difference between marches and marching songs in army practice, albeit the older songs could have incented the instrumental marches.
In former times marches were played by the fifes and drums for the infantry and all other services except the cavalry who were entitled to the royal brass: trumpets and kettle drums; later on all services were entitled to brass bands, even bloody seamen and aviators. German Marches are usually played in 4/4, 2/4, 2/2 or 6/8 (1 pace for every 3 eights).
Marching songs can be traditional folk songs or songs written specially for this purpose, especially with a strong propagandist bias (especially in the 3rd Reich). They are not played by the band, but sung by the troops when marching without a band, or when they've inhaled a good lot of ebriating fluids in the barracks or abroad.
A lot of folk songs in 3/4 were changed to a combination of 6/4 and 4/4. A good example you can find at my site http://www.uni-giessen.de/~gb1053/mudcat.htm#mond. This song was originally tradited in 3/4 and transformed for the howling of marching grunts (did it myself).
Standard reference work about history and collections of German Army Marches is:
Armeemärsche / Joachim Toeche-Mittler. - Stuttgart : Spemann, 19XX [3 vols., 1966-1975]
Standard collection in the "Prussian Army and the Federal Contingents" [official name of the so called "Imperial German Army"]: Heeresmärsche I-III. They are divided according to the different purposes: Parade, Present Arms, Defilee. Mounted troops also had marches for pace, trot and gallop.
German marches were influenced by the most valiant enemy in centuries XVII-XVIII, the Turks. Beethoven's 3 famous Turkish Marches are still played by Bundeswehr bands, and Mozart, Haydn and others are known to have written Turkish marches in their time.
Enough now; I'm just longing to strap my pistol again and start to march before my Federal Reserves platoon as in former times, but alas ... medical discharge.

Wilfried