I want to put the players in a challenging spot by putting them in a no win situation and having them captured and stripped of their equipment (assuming they might get it back as they make their escape, so as not to complete make the players mad.)
In crafting this no win situation, I did not foresee a way out that the players found through some creative genius. Something like instead of focusing their efforts on attacking the huge army in front of them the focus their efforts on luring them into a forest and bringing down the trees to crush them, or focusing on creating a rock slide again to crush them or at least block their path. (maybe not good examples but hopefully you get the point.)
Do you rearrange your story arc for creative thinking? Or do you crush their hopes and force them into the story arc you had originally prepared? Or probably the best route, as a GM hope they don't see a way out but prepare a secondary story arc if they manage to wiggle their way out?
Best Answer
If this is not a natural consequence of previous actions then you are exercising referee fiat over the player's actions which annoys players to no end.
The nature of roleplaying game means that the players are always doing unexpected things. Where the referee creativity comes in is planning out the consequences of what the players do given the circumstances and the NPC's motivations. Not in writing a story that the players ride along with.
So it doesn't matter what the results of the encounter is before it happens. What important is afterwards when you decide how the NPCs and the environment react and change. So the question here what are the consequences of the army's survival and what are the consequences of it's destruction.
Your story arc needs to be a plot arc and it is a plan of actions that will change after the PCs interact with each encounter. You have to think of your setting, NPCS, and locale as a bag of stuff. What mix get put out depends on where your PCs are, and what they did. Do this and you don't have to worry that a specific outcome must occur at a specific time.
The easiest way of planning the plot is to think of it as series of What Ifs. What if Darkon the Lich gets killed here, what if he survives? You need to do this down the line.
While it appears to be daunting in practices these series of what if turn out to be somewhat limited because of interests of the players, how they play, and what they would reasonably do. You can use this knowledge to manipulate the players as well to narrow the range of possibilities you have to prepare for.
Remember not everything is fluid either. For example if you had an natural eruption volcano planned for two months into the campaign then it will generally go off. It going to be your call as to which part of the plot's timeline is like this. Also no wins are fine, if the players have plenty of warning. If two sessions in a 13th century campaign about medieval europe they decide to go and get Genghis Khan's head then it going to be highly probable it is the PC's heads that will wind up on the heap. But the saving grace of the situation is that likely the PCs will hear about the size of Mongol army and the nature of the Khan's guard well before they get to the point of no return. Put out the information and if they ignore it, let the dice fall where they roll.