The Pequod Review:

Catch-22 is commonly believed to be a Vietnam anti-war novel, but in fact it is set in World War II and its targets are much broader: bureaucracies (especially military/political bureaucracies), the principal/agent problem, and the nature of heroism. 

The book centers on a US Army squadron stationed on a small fictional island off the coast of Italy. The group’s commander, Colonel Cathcart, is so desperate to impress his superiors that he continuously raises the number of required missions his soldiers have to fly to such ridiculous levels (topping out at 80 missions) that they never meet the service requirements necessary to earn their transfer back home. The book’s main character is Captain Yossarian, who has flown 50 bombing missions and fears for his mental stability, but is unable to obtain a transfer: 

"Because he's crazy," Doc Daneeka said. "He has to be crazy to keep flying combat missions after all the close calls he's had. Sure, I can ground Orr. But first he has to ask me to."

"That's all he has to do to be grounded?"

"That's all. Let him ask me."

"And then you can ground him?" Yossarian asked.

"No. Then I can't ground him."

"You mean there's a catch?"

"Sure there's a catch," Doc Daneeka replied. "Catch-22. Anyone who wants to get out of combat duty isn't really crazy."

There was only one catch and that was Catch-22, which specified that a concern for one's own safety in the face of dangers that were real and immediate was the process of a rational mind. Orr was crazy and could be grounded. All he had to do was ask; and as soon as he did, he would no longer be crazy and would have to fly more missions. Orr would be crazy to fly more missions and sane if he didn't, but if he was sane he had to fly them. If he flew them he was crazy and didn't have to; but if he didn't want to he was sane and had to. Yossarian was moved very deeply by the absolute simplicity of this clause of Catch-22 and let out a respectful whistle.

"That's some catch, that Catch-22," he observed.

"It's the best there is," Doc Daneeka agreed.

The book is full of funny and absurd moments like this, with madness and folly running through every exchange. Here is Youssarian again: 

Yossarian says, "You're talking about winning the war, and I am talking about winning the war and keeping alive."

"Exactly," Clevinger snapped smugly. "And which do you think is more important?"

"To whom?" Yossarian shot back. "It doesn't make a damn bit of difference who wins the war to someone who's dead."

"I can't think of another attitude that could be depended upon to give greater comfort to the enemy."

"The enemy," retorted Yossarian with weighted precision, "is anybody who's going to get you killed, no matter which side he's on."

The novel’s primary flaw is that it generally maintains this single-note form of sarcasm throughout, with jokes that feel repetitive, and with too large a collection of exaggerated characters. But at its best, especially toward the end of the book, it strikes a deeper chord as the emptiness and tragedy of the war become clear.