As already stated, the first of Mackensen's huge shells began bursting over the Serbian defenses across the river on September 20, 1915. While the wheels of diplomacy continued turning during the following weeks, the roar of the big guns grew louder and more persistent and swept up and down the long line. Then came several attempts on the part of the Austro-Germans to cross the rivers; all these the Serbians successfully repulsed, though they may have been mere feints, as a boxer jabs at his opponent's jaw while he really aims for his wind. There were seven of these attempts. In one, near Semendria, the Serbians reported that a whole battalion of an enemy was destroyed. Meanwhile German aeroplanes whirred back and forth over the Serbian lines, reconnoitering their positions and sometimes dropping bombs. One of them flew south as far as Nish, then turned eastward and disappeared over the mountain ridges toward Bulgaria. And all this while the frontier guards reported that the Bulgarians were massing their troops day by day. As already noted, the Serbian frontier in Macedonia was left practically unguarded. Possibly the Serbians still hoped the Greeks would hold to their treaty and join them from that direction. And, indeed, the Greek army was being mobilized, frankly to meet the Bulgarians. More encouraging still, the news came that France and England, at the request of Venizelos, had agreed to send to Saloniki 150,000 men to make up for an equal number which, by the terms of the Serbo-Greek treaty for mutual defense against Bulgaria, Serbia would have provided had she been able to do so. This force began landing in Saloniki on October 5, 1915, but on the same day Venizelos was again compelled to resign by King Constantine, who was determined to keep the Greek nation out of the war. This was a sad blow to the hopes of the Serbians. Still, the British and French troops continued landing, in spite of the "protest" from the Greek Government. Beginning on October 3, 1915, the fire of the Austro-German artillery became doubly insistent, thundering up and down the whole front with increasing vigor. Again the Teutons began poking their pontoons out into the river, and again they were smashed by the Serbian guns. The fighting waxed hottest at Ram, Dubrovitza, and Semendria, on the Danube, and in and about Ciganlia Island (Island of the Gypsies), at Obrenovatz, Shabatz, and Jarak on the Save, where it is joined by the Drina. Ram and Semendria, both fortified places, guarded the mouth of the Morava Valley, and these Gallwitz subjected to an especially heavy fire. By October 5, 1915, the shelling became heaviest in this sector: the enemy's guns and howitzers belched forth a steady hail of big shells. Belgrade, also, became the object of an increasingly tremendous effort on the part of the Austro-German artillery. Here they had brought up long-range guns, and with these inflicted heavy damage.