A lot of the answe are reddit-esque throwaway answers and political shill bots, but for a more nuanced answer. For a long timeant people were brought up on the "End of History" idea. That after the Cold war mankind would all get together and sign kimbaya, that war as we knew it was over, sure there would be atrocities and "policing actions" that would be needed to stop some bad guys but everything was good wars of the past were over and it was all good.
This started to unravel as things got more tense with the increasing militarization of China and centealzatuon of power in Xi as well as well as Putins more aggressive moves.
The Ukraine war kicked the whole structure over and dillusioned a lot of people. The problem is that if suddenly history isn't over and war is a possibility again you need to start preparing for it. If you choose not to play the great game you don't avoid it, you just get crushed, and so as a matter of national survival and security you need to start preparing to meet near peer adversaries, but they see you preparing and they up their defensive preperations and military spending because you did and if they don't they might be crushed, and you see this and have to up your spending, and suddenly a massive amount of wealth is going to preparing for conflict.
The whole situation is almost identical to how things were on the eve of the 20th century, with radical advances in technology overturning decades of military theory and strategy, everyone is tense with alliances constantly shifting and real politick dominating everything because the alternative is the fate of the Belgians who had their country shorn from them and their people massacred by Imperial German forces. At the same time we see growing social unrest within nations (stoked by foreign actors trying to disrupt social cohesion of their potential enemies) and everyone posturing with more and more bluster because to be precieved as weak is to invite death as no one wants to make friends with a weak link.
The one important lesson to be taken however is that WW1 didn't have to happen the Great War could've been prevented, a major part of the conflict was the certainty that there was going to be a war. If we can choose to instead try and be peacemakers and bring the overall temprature of discourse down we can avoid another human catastrophe.