Johnston had ordered Zollicoffer not to cross the river but Zollicoffer acted rashly and advanced his entire command with the intention of defeating Thomas. When Crittenden arrived on the scene it was too late.
If Zollicoffer had obeyed orders and never advanced, Johnston's eastern flank would probably have remained intact as Buell and Thomas were pretty cautious. However, the Union advance on Fort Henry and Donelson was the event that broke Johnston's tenuous hold on Kentucky.