Situational irony is irony that occurs when something happens that is different from the expected outcome. In "The Machine that Won the War," this happens a few times as readers discover how the three main characters treat the supercomputer named "Multivac."
The humans in the story have been at war with the Denebians for a very long time. They feel that the only sure way to victory is to put their fates into the "hands" of a supercomputer that will analyze the enemy's tactics and come up with its own battle plans and countermeasures to win the war. Based on this initial setup, a reader would expect that Swift, Henderson, and Jablonski are all going to talk about how amazingly the computer executed its job.
That is when the irony of the situation starts to come through. Each man admits that he didn't pay attention to the computer at all. Henderson admits that the field data coming in was so biased that he couldn't trust it; therefore, he simply uses his own intuition and guesses at what data should be fed into Multivac. Jablonski admits basically the same thing. Before the war was over, he realized that Multivac was not working properly, and the reports given by the machine weren't credible. Jablonski then guesses at what information should be passed to Swift. Finally, Swift admits that he never trusted Multivac in the first place. When it came time for him to make an important decision, Swift flipped a coin. While readers begin the story expecting that a supercomputer was integral to winning a war, they are surprised to find out that the war victory was ultimately won through human intuition, guessing, and blind chance.