It could either be devastating for FedEx, or they have something else going for them... and hopefully it's nothing in the tune of a Chinese Investment.
I think it's a bit of a gamble. FedEx is tacitly acknowledging Amazon as an upcoming threat in the logistics segment. If FedEx plays it right, they're an appealing way for companies to compete with Amazon. If they called it wrong, they handed the US market, at least short term, to UPS on a silver platter.
Amazon has its hands in too many pies. If FedEx breaks off, they can still make money elsewhere (ie: shipping buttloads ofonline merch from retailers other than Amazon/shipping across the country or overseas, or sponsoring sci-fi shows). If Amazon makes everything Amazon brand and something happens to Amazon, [insert atomic bomb gif and crash sound]
I suspect once Amazon fully immerses itself into the logistics business it's going to find margins exceptionally slim when it wants to maintain it's free shipping prime promise.
"Napoleon attacked by rabbits" so I had to go google search it. "The story of Napoleon being "defeated" by rabbits during a hunt in 1807 is a humorous anecdote, but it's not a confirmed historical fact, though it is mentioned in General Thiébault's memoirs. "