As the big game dwindled in the mid-1850s, the Lakota began to enlarge their hunting grounds and "ignored the boundaries" of the 1851 treaty. They crossed Powder River (the dividing line between the Lakota territory and that of the Crow) and launched their "own program for expansions" westward at the expense of other Natives. For the Crow, the plains near present-day Wyola, Montana became a field for "large-scale battles with invading Sioux". By 1860, the Lakota and their allies had driven the Crows away from their treaty-guaranteed hunting grounds on the west side of Powder River.