Anarchists can organize and fight militarily quite effectively while still maintaining a bottom-up system, as demonstrated by the Anarchist militias of Makhno and Catalonia (and though not Anarchist, Rojava is an example of a decentralized army). The issue all Anarchist attempts have faced militarily is a lack of any allies on the world stage, allowing the authoritarian nations around them to crush them due to lack of supplies (or sudden betrayal of ML 'allies').