Democracy and liberty are often confused. Democracy simply means rule by vote. Liberty means that even in a democracy (or a democratic republic) there are some decisions that the majority cannot make. The decision to sacrifice one's life in a war is one of those decisions.
In other words, your life is your own, and not the property of 51% of the population.
We the people of the United States, in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity