Situated as it was near the Mexican border, the ranch of Diamond X was near the head of a great valley--a natural pass between the two countries. Through this pass flowed Spur Creek, branching out into one or more streams in different places. You probably know that to successfully raise cattle, horses or sheep two things are needed--food and water. Food is supplied by the various rich grasses that grow naturally on the western plains. Water is not so plentiful in that sometimes arid region, and for that reason is jealously guarded. A ranch with a natural water supply is worth ten times what one is without fluid for the cattle to drink. Driving herds long distances to quench their thirst runs off their fat, and as cattle are now sold by the pound, instead of by the piece, as formerly was the case, the heavier a steer is the more money he brings. Spur Creek, then, was a valuable asset to Mr. Merkel, and he determined to fight for it to the "last ditch," so to speak. This water was only a part of the courses that were valuable to his ranch. As for the boys, they had a water supply of their own in Happy Valley, though they had had to fight to secure that, as related in the book named "The Boy Ranchers in Camp." "Well, if there's to be a fight, the sooner the better," commented Bud as he and his cousins washed up at home after their night in the open. They told of their experiences, which really amounted to nothing as far as getting a trace of the fugitives was concerned, and then. Mr. Merkel sent word to Sheriff Fowler of the theft. "And now we'll build a fort at Spur Creek," said the ranchman. "A _fort_!" cried Bud. "Well, it will be a sort of fort," his father went on. "There is one place there just right for defensive operations and we'll put up a shack there and mount guard until the danger is over. Once the sheep men see that we mean business they may throw up their hands and go back where they belong--in Mexico." There were soon busy times at Diamond X. The flivver was called into requisition, and on it and on wagons was transported to Spur Creek lumber to make a rough shack as a shelter for those who would be kept on guard against the advance of the sheep herders. "And we're going to form part of that guard!" declared Bud. "Our ranch can run itself for a while. We've got to stick by Dad!" "That's right!" agreed Nort and Dick. Secretly they rejoiced at the chance of a coming conflict, even though they had so recently had a hard time campaigning against the Yaqui Indians.