When your life depends on it, you never want to count on another branch of the service. Or you wind up asking for help from a douche like US Army General McChrystal. Who won't give it to you. Here's one person's comments on how the general fucked over some Marines and Navy:

   they left our men to perish without fire
   support because of your rules of engagement

   I don’t retreat one iota from what I said there.
   McChrystal, with his ROE, is a murderer

The request for help went all the way to General McChrystal, but he wouldn't give it to them. Asked again, he refused again.

http://www.captainsjournal.com/category/general-mcchrystal/

http://www.captainsjournal.com/2011/02/17/reprimands-in-mari...