on the defensive
Prepared for withstanding aggression or attack, as in The debate team's plan was to keep their opponents on the defensive, or This teacher put students on the defensive about their mistakes. [c. 1600]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.