In The Watchmen, characters face many challenges and choices that affect their relationships and the fate of the world. However, a certain recurring idea is entangled in their journey, given in imagery, depicted through the setup of the story, and debated through the internal struggles of characters.
This idea is the conflict between fate and free will. The concept of fate and predestination is especially contemplated by the character Doctor Manhattan, who essentially gains an understanding beyond time, while his existence is still limited to linear experiences.
Doctor Manhattan can look upon events in the future as we look upon the past; he can't change what has happened or will happen, but he still can regret and try to learn from them. This contributes to conflict because he already knows what people will do, creating a distance between himself and others as he is not always mentally present in the moment.
Furthermore, the story itself contributes to this debate by placing events of the "future" next to the events of the present or past. It tells itself in a way that makes the future seem inevitable, building on little events or actions that snowball into huge plots. Details shown in the beginning recur and build on the idea that everything that happens is predestined in a universal domino effect.
This idea is also present throughout the novel in small details such as the clock at the beginning of every chapter. Representative of the doomsday clock, it slowly counts down, once again reinforcing the idea that events are always building up to an inevitable and destructive conclusion.