Putnam. 540 pp. $19.95

TOM CLANCY's first two books were not so much novels as extended commentaries on war games -- which I mean as a compliment, of course, not a complaint. In sitting down to play out Midway, I can't think of anyone else I'd rather have at my elbow, offering advice, except Admiral Spruance himself. But in both those early books, the deck was very much stacked in Clancy's favor. The map board was in place, the pieces arranged, the rules well established. Again, there's nothing wrong with that, but it's interesting to see how Clancy has fared now that he's designed a game all on his own -- for Patriot Games is about terrorism, where there are no rules, no uniforms and no set-piece battles at all.

Things begin well enough. Jack Ryan, Clancy's previous hero in The Hunt for Red October, is walking across St. James's Park in London when a splinter group of the IRA attemps to snatch Prince Charles, Princess Diana and Prince William. There's a lot of shooting and blood, but Jack is a good, decent man and an ex-Marine and so he plunges right in and saves the day, despite taking a bullet in the shoulder for his trouble.

A spirited opening. But immediately the book bogs down: we are forced to suffer, along with Jack, through 200 pages of his hospitalization and convalescence. Indeed, all that enlivens this section of the novel is the Royal Family. Of course, they have to be brought in -- Jack, after all, has saved two princes of the realm. Graciously, Her Majesty knights him (leaving Maggie and Ron to sort out the legal technicalities) and then lays on a special tour of the Tower of London. This is ludicrous, of course, but it could be worse; you can more or less keep Prince Charles and Prince Philip separate in your mind as you read.

Clancy's difficulty here is that he attempts to assimilate the Royals to American values through the locker room -- Charles is an all right guy because he flew Phantoms -- and the sitcom: Elizabeth and Philip are old married hands straight out of Father Knows Best, while Di is happily preggers again. This is fine, as far as it goes, but it overlooks the crucial fact that these people are Brits. Clancy doesn't seem to understand that royal gratitude, in such circumstances, would be genuine enough, but certainly wouldn't extend to wanting Jack as a friend.

Still, he finally escapes them, and returns home to Annapolis. There, after a few cloying scenes of suburban bliss -- Jack's wife is also expecting -- the bracing American air finally gives the plot some get-up-and-go. The terrorists, it seems, are intent on revenge, and when Charles and Diana, on an American tour, drop by Jack's place for barbecue, they take their chance. Soon, we're back in a world of Redeye missiles, Browning automatics and police car chases. On this ground, few people can beat Clancy, and he gives us a bang-up climax with plenty of helicopters, dead bodies and a rousing sea chase -- the prince, you'll be happy to know, acquits himself well.

THERE ARE two problems with this. The first is literary. In the end, Clancy is not quite sure which game he wants to play. Every once and a while, Patriot Games tries to be a spy story, but the intrigue is so feeble and artificial that Clancy quickly gives you the solution to his mysteries and reverts to the pyrotechnics of the adventure yarn. Here, he's more confident, but by dividing his attention he prevents the action from ever gaining momentum.

The second problem is even greater and yet, in a curious way, it saves the book. I can't remember when I last read a novel so politically naive. Clancy apparently subscribes to every single myth in which authority cloaks itself. They're all here: God, the home, the family; law and order; friendly British bobbies and stalwart NCOs -- not to mention Her Majesty and Their Royal Highnesses. Of course, the values embodied in these myths are important enough, but surely Clancy doesn't need to take them so literally. The office of the presidency is no doubt worthy of respect, but I'd still be skeptical of Reagan's news conferences; and though I have the greatest appreciation for the extraordinarily fine, and delicate, tradition of authority represented by the monarchy, this doesn't require me to believe that the Princess of Wales has any more brains than Miss Universe.

Still, one should give Clancy his due. He has the virtue of his vice -- enthusiasm. And though one can argue that this is the most dangerous virtue of all, it is the moving force in his book. I didn't believe a word of Patriot Games, but I certainly believed that Clancy believed, and that's enough to carry you through.

In short: The Hardy Boys Meet the Royals -- well below Clancy's previous efforts, but some pop in the end. Beach bags should be bulging with it all through the summer.

Anthony Hyde is the author of "The Red Fox." His next novel will be "China Lake."