As American soldiers traveled across the ocean to Europe, a ______________ system was used for protection and rescue from German U-boat attacks?

'Cause we got a little ol' convoy
Rockin' through the night.
Yeah, we got a little ol' convoy,
Ain't she a beautiful sight?
Come on and join our convoy
Ain't nothin' gonna get in our way.

Read more:  C.W. McCall - Convoy Lyrics | MetroLyrics