Actually, I think most mistakes in history were on the political level. There have been very few battles and wars that have actually changed much, without the political foreplay having a great importance. Even examples such as Hastings, where it might seem the battle itself was crucial, the politics played a major role. One of the reasons Harold wanted to stop William immediately at the coast was because Harold wasn't sure of his claims for England were strong enough, and was afraid of rebellion if William would start plundering southern England. The punic wars are also an example of how great tactics and fairly good, but not great, strategy can't break an empire with a recruitment system where new armies are always raised whenever one is destroyed.
Actually, name almost any mistake/disaster for any nation/group/people in history and I think I'm able to point out an important political reason behind why it went the way it did. Sometimes mistakes which were hard to foresee, but often possible to avoid with a sound policy. Keep in mind that politics are:
- raising and maintaining an army (i.e. determining it's strength)
- all diplomatic actions
- choosing how to fight a war - whether to accept surrender that would make both sides feel victorious and end the fighting, or stubbornly pushing a right to impose a near unacceptable peace on the vanquished
all these things might seem like military matters, but are usually decided by the politicans/kings/emperors etc., not generals
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