In Watchmen, Ozymandias plans to avert global war and bring about some level of world peace
by faking an alien attack on New York.
The only potential obstacle to his plan is Dr. Manhattan. So he creates a tachyon generator to obscure Dr. Manhattan's perception of time. Then, Ozymandias lures Dr. Manhattan into a trap and removes his "intrinsic field," disintegrating him.
However, it was an accident involving intrinsic fields that turned Jon Osterman into Dr. Manhattan in the first place, and he quickly reassembles himself and confronts Ozymandias.
Reassembling myself was the first trick I learned. It didn't kill Osterman... did you really think it would kill me? ...The world's smartest man poses no more threat to me than does its smartest termite.
We've also seen that Doctor Manhattan can modify his body at will, teleport, and even create duplicate bodies for himself, so it seems pretty obvious that this trick won't stop him. Why did Ozymandias, the world's smartest man, think that it would?