Naval history from first to last is full of illustrations of the principles here expounded. I will examine one or two of them, and I must take my illustrations mainly from the naval history of Britain, first, because Britain, being an island, is the only country in Europe which cannot be invaded except across the sea, and secondly, because Britain for that very reason has often been subjected to attempts at invasion and has always frustrated them by denying to her adversary that sufficiency of sea control which, if history is any guide, is essential to successful invasion. But first I will examine two cases which might at first sight seem to militate against the principles I have enunciated. The brilliant campaign of Cæsar which ended in the overthrow of Pompey and his cause at Pharsalus, was opened by Cæsar's desperate venture of carrying his army across the Adriatic to the coast of Epirus, although Pompey's fleet was in full command of the waters traversed. This is one of those exceptions which may be said to prove the rule. Cæsar had no alternative. Pompey was in Illyria, and if Cæsar could not overthrow Pompey on that side of the Adriatic it was certain that Pompey would overthrow Cæsar on the other side. For this reason, and perhaps for this reason alone, Cæsar was compelled to undertake a venture which he must have known to be desperate. How desperate it was is shown by the fact that, not having transports enough to carry more than half his army at once, he had to send his transports back as soon as he had landed, and they were all destroyed on their way back to Brundusium. Antony his lieutenant did, indeed, succeed after a time in getting the remainder of his army across, but not before Cæsar had been reduced to the utmost straits. The whole enterprise moreover was not, strictly speaking, an invasion of hostile territory. The inhabitants of the territory occupied by both combatants were neutral as between them, and were willing to furnish Cæsar with such scanty supplies as they had. Again, an army in those days needed no ammunition except the sword which each soldier carried on his person, and that kind of ammunition was not expended in fighting. Hence Cæsar had no occasion to concern himself with the security of his communications across the sea--a consideration which weighs with overwhelming force on the commander of a modern oversea expedition. "A modern army," as the late Lord Wolseley said, "is such a complicated organism that any interruption in the line of communications tends to break up and destroy its very life." An army marches on its belly. If it cannot be fed it cannot fight. After the Battle of Talavera Wellington was so paralysed by the failure of the Spanish authorities to supply his troops with food that he had to abandon the offensive for a time and to retreat towards his own line of communication with the sea. Cæsar on the other hand abandoned the sea, which could not feed him, and