I wonder when the first corporation will declare war?  It's not that inconceivable.  Guns and ammunition are already made by private companies.  And the U.S. has used private military contractors in the recent past.  So the equipment and skill is there. 

There's certainly a question about legality, but I feel like that's not all that hard to circumvent.  It probably won't be much longer before the first company establishes their corporate headquarters on an oil rig in the middle of the ocean.  It's kind of already happening.  It's not that much of a stretch for a "service economy" company like Google to simply build a few floating platforms in the world's oceans and continue to rake in cash while also continuing to avoid paying taxes.  I'd say the incentive is right there. 

The final thing would be motive, and I think a perfect example of that is the recent Sony hack.  A giant corporation has little or nothing to gain from a legal investigation and subsequent trial.  But they have everything to gain by preventing hacks from happening in the first place, both by increasing their network security and by personally attacking offending parties.  Plus, it wouldn't take a D-Day invasion to take down a group of computer hackers.  It would take a Seal Team 6 type of group, but without the legal niceties and Geneva Conventions that sovereign nations have to follow.  Businesses are already considering this. 

In short, the next major development in warfare will be Company X vs. Nation Y. 

Update:  Scott Adams wrote the same exact thing a few days after me. #business