While Oracle has talked of all the gains it will make by acquiring Sun, the early benefits, as usual, have gone to competitors. As the Wall Street Journal notes, IBM, HP and Dell have all picked up market share in the server market without paying a dime in acquisition costs and without having to deal with the complex integration that follows a major purchase.

In the long run, of course, it still remains to be seen how the Oracle-Sun deal will fare. But it’s worth noting what’s happening in the short run because companies often underestimate how much market share an acquired company will lose when it is purchased. Mergers put blood in the water, and competitors go on a feeding frenzy. They are more likely to succeed, too, because customers know they have a period of adjustment ahead of them as their supplier gets integrated into the acquirer. If a customer is at all unhappy, it may decide to go ahead and switch suppliers right away rather than wait for problems to pop up during the transition.

Because of the uncertainty caused by the Oracle-Sun agreement, Sun’s market share for servers has dropped to 10% from 11%, and the situation just got worse. The European Union said it will investigate the merger, which may prolong the confusion among customers for months or even quarters. By the time Oracle actually gets to take over Sun–assuming that’s the outcome–Sun may be a much smaller company than it was when the deal was announced.

Did Oracle allow for that contingency? Maybe. But, if history is any guide, Oracle probably didn’t. Others would do well to be more realistic.