They recognize you can defeat them in the field, so they have resorted to the ninja to kill you.
8 seconds sound clip from the Ninjutsu (1968) movie soundboard.
You can hear this line at 00:08:00 in the Blu-ray version of the movie.
Quote context
[...]
- But from the manner in which they fought, I don't think they're Chinese.
- Also, why should they want to assassinate you, sir. Who gains from your death?
- They are known as Ninja, these assassins.
- The Japanese bandits may burn our villages, but they're no match for you.
- They recognize you can defeat them in the field, so they have resorted to the ninja to kill you.
- I hear not to far from here, there lives an old man. In his youth, he used to be a pirate chief.
- He has lived in Japan, and he is well versed in the ways of the ninja.
- But he left the sea many years ago now. Maybe he is too old to help.
- I'd like to meet this old man in person. Sounds interesting.
[...]