U.S. expels 35 Russian diplomats, closes two compounds: official

War is politics by other means. It's a method of forcing a political opponent to take an action or raising the cost for them to take an action. There are several debates around what justifies the need for conflict.

The UN's R2P doctrine (responsibility to protect) states that states have the right to militarily intervene in other states to stop crimes against humanity such as acts of genocide. Some say that that there are moral absolutes to protect (then again some argue that taking political action on moral absolutes is dangerous) or that not intervening early can result in further destabilization (which some argued was a reason Syrian intervention was needed earlier but is too late now). Others argue that imposing a foreign will on others no matter what the circumstance is wrong even if acts like genocide are taking place.

Another argument is the Neville Chamberlain argument. Some argue appeasement made WW2 an inevitability due to it giving Hitler increased political power and military power. Was he a fool for appeasing Hitlers or was he acting the best he could with the knowledge of what war would cost Britain? Again, it's complicated. While I doubt people believe war with Russia or China is inevitable or should happen (then again there are nuts everywhere) there is the argument that attempting to completely rule conflict out would make conflict paradoxically more likely.

Even these two examples are up for books worth of interpretation but the short of it is that war sucks but it happens, like you said, for very complicated reasons.

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