"The enemy of my enemy is my friend." -- Arab proverb.

Reading yesterday's news that Boeing (NYSE:BA) has agreed to assist Japan's Mitsubishi Heavy Industries in selling and maintaining that nation's first postwar commercial jetliner got me thinking: What's in it for Boeing? I mean, after years of merger upon merger, the international aircraft industry has concentrated itself to the point where all Boeing really has to worry about is contending with an A380-hobbled Airbus. Why now turn around and help build up a future rival?

Reviewing the specifics of the firm's collaboration, I've come up with a few theories. According to the Nikkei Business Daily, Mitsubishi is not developing large aircraft that would compete with Airbus' or Boeing's bread-and-butter airliners. Instead, it aims to build 72- and 92-seat aircraft, at least initially. Which current operator in that space would be hurt by new competition?

For the answer, I reviewed the most recent 20-F filing from Motley Fool Stock Advisor recommendation Embraer (NYSE:ERJ), which builds just such aircraft. Embraer divides its operations into three main commercial lines:

  • Small airplanes, such as business jets, in competition with Canada's Bombardier, France's Dassault Aviation, and U.S. firms General Dynamics (NYSE:GD), Raytheon (NYSE:RTN), and Textron (NYSE:TXT).

  • The sub-100-passenger range of aircraft, dominated by offerings from Embraer and Bombardier.

  • Jets carrying passengers into the low 100s, where Embraer and Bombardier begin competing with the duopoly of Boeing and Airbus.

As I see it, helping Mitsubishi Heavy get its commercial airliner business off the ground (pun intended) gives Boeing two advantages -- three, if you count the obvious revenue stream from the partnership itself:

  • First, Embraer's introduction of its Embraer 190 and Embraer 195 lines suggests the company may be moving toward building bigger planes that could rival Boeing's offerings. By bolstering a competitor in the sub-100-passenger space, Boeing increases pressure on both Embraer and Bombardier, perhaps slowing their advance into building larger airliners.

  • Second, if the partnership with Mitsubishi Heavy works out well, Boeing may expand its involvement. In that regard, the partnership promises to facilitate Boeing's entry into this segment of the market, should it so choose at a later date.

Of course, it may be that the obvious theory is the only correct one. I'm just Fooling around with some ideas here. Take them for what they're worth.

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Fool contributor Rich Smith does not own shares of any company named above.