Final answer:
The statement "great armies to spring from the ground and march to and across frontiers" provides evidence that nations did not understand how extensive the war would be.
Step-by-step explanation:
The statement that provides evidence that nations did not understand how extensive the war would be is:
"great armies to spring from the ground and march to and across frontiers"
This statement suggests that nations did not anticipate the scale of the war and the mobilization of large armies crossing borders. It implies a lack of awareness and foresight regarding the extent of the conflict.
Learn more about Understanding the extent of the war