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That would be an unfortunate situation. Many 'Buddhist countries' have and are fighting wars where people who are Buddhists are engaged in warfare.
What can a Buddhist do if he is conscripted? Not all Buddhists are at the same level of virtue, tranquility and wisdom. So it would depend. Some may refuse to kill and harm and face the consequences. Others may seeks vocations which does not involve killing. However, if you are to go to the front
Be mindful
Accept responsibility
Never 'delight' in killing or harming, even an enemy
Contemplate the drawbacks of hate, the ego, self-views (collective and individually)
Once a week keep the Uposatha practice (re: Visakha uposatha sutta)
Practice metta (to one's colleagues and even the enemy, remembering that one could have born into the other side, and be fighting his current comrades.)
Samsara is a dangerous place, there are many times and places where hearing and practicing the Dhamma is impossible. So it is also worth considering to contemplate 'what do buddhists do in situations which are conducive for the Dhamma?'. Many waste such rare opportunities, squander it chasing after sensual pleasures and feeding the ego.