WARNING: Major spoilers ahead for Game of Thrones Season 7. Continue reading at your own risk...
If you ask Jon Snow, the White Walkers are the most formidable force in Westeros. As for Daenerys Targaryen, she would likely say that her dragons have the upper hand. However, both the King in the North, the Mother of Dragons, and everyone in Westeros could be staring down the barrel of an even more urgent catastrophe. The biggest threat the key characters on Game of Thrones are facing is the scarcity of food.
While food may not carry the same amount of excitement that fire-breathing beasts or undead armies, it could be the reason that several major armies that wither away.
Since the beginning of the wildly popular HBO series, scarcity of food has been an ever-present issue. For example, there was the Riot of King's Landing in which the starving population threw cow pie at King Joffrey and actually tore the High Septon apart limb by limb.
In another scene, Littlefinger issues a warning to the Small Council that there is only enough food to withstand five years of winter. This is problematic considering that winter could likely last much longer than a half decade.
Further evidence is when Bronn asks Tyrion and Varys, "Have you ever been in a city under siege? Maybe this part's not in your books. See, it's not the fighting that kills most people; it's the starving. Food's worth more than gold."
In episode four of this season, which was titled "The Spoils of War," the Lannister army was scorched to ash by Dany and her beloved dragon, Drogon. The Lannisters were in the process of bringing sacks of grain back from The Reach to King's Landing when Dany came along and wiped out all the resources.
Up north, in Winterfell, Sansa Stark has been shown expressing her concern for the amount of food available. There's only enough food to feed the current inhabitants of Winterfell for a year. If other armies come to their aid, those supplies will run out at an even higher rate.
"We need to start building up our grain stores with regular shipments from every keep in the North. If we don't use it by winter's end, we'll give it back to them. But if the entire north has to flee to Winterfell, they won't have time to bring wagon loads of grain with them," Sansa says.
With all this being said, each character has their own agenda and issues they are currently facing. However, everyone in Westeros is going to feel the impact of a scarcity of food.