system covered the few available roads. The Struma and Nestos rivers cut across the mountain range along the Greek-Bulgarian frontier and both of their valleys were protected by strong fortifications, as part of the larger Metaxas Line. This system of concrete pillboxes and field fortifications, constructed along the Bulgarian border in the late 1930s, was built on principles similar to those of the Maginot Line. Its strength resided mainly in the inaccessibility of the intermediate terrain leading up to the defence positions. Strategy Greece's mountainous terrain favored a defensive strategy, while the high ranges of the Rhodope, Epirus, Pindus and Olympus mountains offered many defensive opportunities. Air power was required to protect defending ground forces from entrapment in the many defiles. Although an invading force from Albania could be stopped by a relatively small number of troops positioned in the high Pindus mountains, the northeastern part of the country was difficult to defend against an attack from the north. Following a March conference in Athens, the British believed that they would combine with Greek forces to occupy the Haliacmon Line—a short front facing north-eastwards along the Vermio Mountains and the lower Haliacmon river. Papagos awaited clarification from the Yugoslav government and later proposed to hold the Metaxas Line—by then a symbol of national security to the Greek populace—and not withdraw divisions from Albania. He argued that to do so would be seen as a concession to the Italians. The strategically important port of Thessaloniki lay practically undefended and transportation of British troops to the city remained dangerous. Papagos proposed to take advantage of the area's terrain and prepare fortifications, while also protecting Thessaloniki. General Dill described Papagos' attitude as "unaccommodating and defeatist" and argued that his plan ignored the fact that Greek troops and artillery were capable of only token resistance. The British believed that the Greek rivalry with Bulgaria—the Metaxas Line was designed specifically for war with Bulgaria—as well as their traditionally good terms with the Yugoslavs—left their north-western border largely undefended. Despite their awareness that the line was likely to collapse in the event of a German thrust from the Struma and Axios rivers, the British eventually acceded to the Greek command. On 4 March, Dill accepted the plans for the Metaxas line and on 7 March agreement was ratified by the British Cabinet. The overall command was to be retained by Papagos and the Greek and British