Translation:During the Ten Years, the leader won battle after battle.
The Tien Jaren is the name of a period in the Tachtigjarige Oorlog, during which things went exceptionally well for Prince Maurits. See https://nl.m.wikipedia.org/wiki/Tien_jaren_(Tachtigjarige_Oorlog) . It was not a separate war.
Slag is more general, and although it is clear that it means veldslag ('battle on a field') when used capitalized and with a place name as in 'Slag bij Nieuwpoort', it can just mean 'beat', 'slap', 'strike', 'swing' or 'hit' depending on the context.
Some other compound words with the 'slag' element: donderslag, paukenslag, blikseminslag, zeeslag, slagwerk. (thunderclap, hit/strike on kettle drum, lightning strike, sea battle, percussion instruments)