British and French generals tried to solve the problem by novel uses of their infantry and artillery, but to no avail. Concentrating sufficient artillery and other forces to promise a breakthrough meant alerting the enemy to the imminent attack. This was especially so since the German positions in France lay along ridges of hills looking down on Allied forces in the flat plains below. The Germans could always prepare, not only bringing in reinforcements but sheltering their defending forces in deep, elaborate underground chambers. An attack in a limited area, where available heavy guns could be used in concentrated fashion, invited the enemy to respond by concentrating his reserves in this small segment of the front. An attack over a vast section of the front spread the available artillery too thinly to have much of an effect. Some political leaders wondered whether there were other places to launch a decisive operation besides the densely defended western front. Winston Churchill was a fervent advocate of seizing German coastal islands in the North Sea such as Borkum or Heligoland, and Admiral Sir John Fisher, Britain's first sea lord, favored a landing on the Baltic coast. Such ideas dissolved upon close examination. The strength of German coastal defenses, and the vast mined areas in these seas, put any unit of the Royal Navy involved in such adventures in mortal danger. Apart from the peril of sending a naval force into the Baltic, the speed and efficiency of the German railroad system meant that a landing force would be outnumbered almost instantly and wiped out soon after that. Early in the war, Lloyd George took up the cry for an offensive thrust into the Balkans. Indeed, the successful Allied offensive there in the fall of 1918 had vast strategic consequences, one of them being to expose southern Germany to invasion. But for most of the war, the prospects in this region were dismal. In 1916, 300,000 Allied troops were held firmly in the bridgehead at Salonika in what the enemy called "the greatest Allied internment camp." Attempts to move northward failed repeatedly so long as the Bulgarians, buttressed by the Germans, remained firm. Cadoma's repetitive efforts to break through Austro-Hungarian defenses on the Isonzo from 1915 to the last months of 1917 showed how little could be done in such terrain.