Killing is bad, hence wars brings a lot of destruction to land,
properties, lives and the eco-system and even when the war is over,
it will bring ill-will to those conquered or vanquished. look at
the animosity of china or s korea over japan.
The ruler of the kingdom or head of state will bear the
responsibility of starting the war, they will have accumulated
untold bad kamma as a result like after they die, they will go to
avici hell for untold eons or kalpas.
Personally I feel that head of state shoud do their best to avoid
wars, cos war bring untold sufferings to its people and the
economy. But if another country is to wage war, no choice but to
defend, but remember to treat POW's with humane treatment, not like
torturing them for information or retribution , adhering to the
Geneva conventions on war.
Buddha once stopped a war between the warring states, think was the
shakyans over water on a river. The Buddha stood at the scene of
the battleground and as a result, the warring states gave respect
to the Buddha by not going to war. The Buddha asked the respective
sides, what was the value of water, they replied value of water is
little. The Buddha then asked, what's the value of blood, they
replied it was invaluable. Such was the wisdom of the Buddha, that
he helped averted a war.
Hope the current and future leaders or head of state will adopt a
commen sense approach and not go to war for oil, minerals, money or
whatever you can think of.