Not all resources will have to be depleted, only scarcity of critical ones, often abruptly.
I was infantry but for a first world country. When we look at the conflicts that we've had against third world countries, we see the lack of logistics and how little people can fight on. Afghani, Japanese and Vietnamese soldiers were regularly fed with only minimal rations, usually just rice and whatever else they can scavenge from their surroundings if not stealing them from the local populace (WW2 on both sides.)
For a non-military collapse (and I think the best example given our reliance on oil), Britain nearly had a societal collapse when their oil refineries were blocked by protestors in the early 2000s. Fights were breaking out at gas stations, people couldn't get to work, public transportation had no fuel, medical equipment and food couldn't be delivered, etc including products from petrochemical production. That was only for a few days too.o Now imagine if/when potable water becomes scarce.
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@GON states, people/societies with less or nothing to lose will do desperate things when their back are against the wall with little to no options. An event that comes to mind is the airline crash in the Alps(?) where the survivors went an extended period with no food and had to resort to cannibalism to survive until rescue.
As intelligent as humans can be, are still animals and will resort to herd-like mentality. A little disruption of common resources can bring disorder; and when citizens start to stockpile supplies that are in short supply can exasperate the issue. We have governments and officials in place that are supposed to plan for catastrophic events but as we've seen lately a self-serving government is little of no help and multiple catastrophic events of different categories can quickly bring down a complex society (bronze age collapse.)
