At the time of the Frankish invasion of Gaul, these practices were found in operation, and as they corresponded in their main features to customs current among the Franks, the German customs and the Roman customs merged the one in the other and in their new form were adopted by the invaders. The coupling of the special obligation of military service as a condition of land tenure was strengthened by the efforts of Charles the Great. The growth in size of the Frankish empire, resulting in campaigns being necessary at great distances, produced a modification of the existing practice. Of special significance was his ordinance that the vassals should come into the field under the command of their lords; as a result, each lord endeavoured to secure as fine a body of vassals as possible. Gradually it thus came about that the inherent duty of the citizen to defend his country "was transferred from a public obligation into a private contract." The Feudal System developed further when other functions of the State passed into the hands of individuals. Of great importance in this connection was the acquisition of the power of "jurisdiction," by which the administration of justice passed out of the power of the State so far as persons residing within the limits of the fief were concerned. Thus it gradually came to pass that all real power passed from the State and centred in individual lords with the result that patriotism and a common national feeling were almost entirely wanting. Yet, from the very time of its origin, the Feudal System contained within itself factors which influenced its decline and fall. The only force that held together a fief was the personal ability of the successive generations of lords, coupled with the nature of their success in maintaining order and security and in compelling outlying landlords to recognise their supremacy. But vassals were ever ready to throw off their allegiance and to assert sovereign rights, if the opportunity occurred, and neighbouring great barons would not scruple to entice the vassals of a rival to change their over-lord. When the Feudal System became fixed, such things might become less frequent, but, generally speaking, the law of the survival of the strongest prevailed.