Huddled close in a thicket, they consulted the radio compass that Frank drew from his pocket. That gave them the general direction in which they must go. They knew that in general their course led toward the west, but, as they could not tell what changes had taken place in the position of the armies as the result of the two days' fighting, they had no idea of how long it might take them to reach the American lines. They got their bearings due west and set off. They were making fair progress when they were startled by hearing the clatter of hoofs a little ahead of them. "Listen!" hissed Bart. "It's a cavalry troop," whispered Frank, as he flattened himself behind a bush, an example that was promptly followed by the others. "Troop!" growled Tom. "It sounds more like a brigade." "Uhlans, probably," conjectured Billy. They peered through the bushes at the broad road not more than twenty feet away. At that moment the moon showed a slender rim above the horizon and threaded the darkness with a faint shimmer of light. Along the road came a force of cavalry. The guttural voices of the riders told the concealed watchers that they belonged to the enemy. In the dim light they could see the steam that rose from the horses' flanks. Those days had been the first for a long time that cavalry could be used on the western front. Trench fighting had put that arm of the service almost wholly out of action. But the fact that the Allies had followed up their tank attack with cavalry had brought forth a German response of the same nature. There was no sign of elation among the riders, and the boys drew pleasure from that. A dejected air prevailed, as though the Uhlans had had the worst of it. "Guess they've had the hot end of the poker," whispered Bart. "Looks like it," replied Frank. Something just then frightened one of the horses, and he reared and plunged into the bushes at the side of the road. The boys had all they could do to scramble out of reach of the iron-shod hoofs. The rider was almost unhorsed, but managed to retain his seat and quiet his trembling mount. By the time he had done this, the troopers had almost passed. The boys were rejoicing at this, but their exultation changed to uneasiness when the soldier who had had so much trouble rode up to an officer and began to talk volubly, at the same time pointing toward the bushes. "Here's where I see trouble coming," muttered Tom. "He's on to us," agreed Bart. "He must have seen us when we got out of his way," said Frank. "Let's get out of here, quick." But this was not to be done so easily, for even as he spoke the officer rapped out a command and a group of twenty horsemen began to spread out and surround the place where the Army boys were crouching. To remain there would be fatal, for it was only a matter of a few minutes before that ring would close upon them with a grip of iron. At all hazards they must break through.