This article seeks to analyse the presence and activities of Lithuanian grand ducal power along the River Nemunas in the period 1283 to 1410. The war between the Teutonic Order and the Lithuanians is viewed from the point of view of challenge-and-response theory. A detailed analysis of narrative sources has allowed us to distinguish two periods in which Lithuanian grand ducal power actively promoted the introduction of innovations in the Lithuanian art of war. The first period encompasses the last decade of the 13th century and the first decade of the 14th century. In this period, not only was a line of Lithuanian castles put in place along the rivers Nemunas and Jūra, but also what we call the Lithuanian military riverine fleet was created. The period was also likely to have been a time when Lithuanian forces adopted the crossbow. The second period involves the last two decades of the 14th and the early 15th century. In this period, a more active defence of fords across the rivers Nemunas and Neris was undertaken from time to time by Lithuanian troops, by putting up wooden fortifications and employing artillery. The synergy of fortification and artillery was a recipe for Lithuanian troops to counter some of the advantages enjoyed by their Teutonic adversaries on water and on land.