What does 'to place at the mercy of' mean?

I'd be extremely grateful for the definition of this phrase!

"It seemed a tragedy that the very discipline that made the Defenders famous now placed them at the mercy of their enemies."

Question

What does 'to place at the mercy of' mean?

I'd be extremely grateful for the definition of this phrase!

"It seemed a tragedy that the very discipline that made the Defenders famous now placed them at the mercy of their enemies."

Answer

To be at the mercy of something means essentially to be helpless while facing it.