What is the purpose of the army?

As spelled out in Title 10 US Code, Section 3062 (a): an Army that is capable of preserving the peace and security, and providing for the defense, of the US, supporting national policy, implementing national objectives, and overcoming any nations responible for aggresive acts that imperil the peace and security of the US. (Paraphrased) It's a big job and it covers a lot of ground. or the British 1 to protect the UK mainland, coast and there Territories. And to keep peace in warring nations.