Leodiensibus copiis is ablative. The phrase is likely to mean something like literally "and therefore to equip himself/herself/themselves with troops from Liège", i.e. to enlist troops from Liège or something along those lines. Though I can't be 100% sure of the meaning of copiis without context, troops or armed forces seems the most likely at first sight. As context tends to be important for interpretation it's usually a good idea to post at least one complete sentence even if your question concerns only a part of it.