What do buddhists do in a situation of war where all able-bodied men must become soldiers? I was reading a chapter called "The Buddhist Stand on the Death Sentence" on p. 390-393 in the book "What Buddhists Believe" by Ven. K. Sri Dhammananda. In here it is written that the law is supreme and that buddhists must always respect and obey the laws of the country they live in and that if they break the law they must also accept the punishment.

Then in the end of the chapter he talks about the situation where war breaks out and all able-bodied men are to be conscripted as soldiers by law of that nation. 

Being a soldier requires one to maybe engage in killing of other beings or maybe become forced to commit unwholesome actions that one does not want to do since it will break the buddhist ethical code and precepts. But if they choose not to do it they will be breaking the law of that nation.

What do buddhists do in that situation? 

Thank you for your time.

Lanka
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.