War can impact societies in many different ways. Many of the impacts it can have are negative, but it can have some positive impacts as well.
The most obvious impact that war can have on society is the death and destruction it causes. Any country where a war occurs suffers greatly. Its economy is devastated as are social bonds between people when, for example, family members are killed. Even a society that simply participates in a war where the fighting is done somewhere else (such as the US in World War II) can suffer serious social disruption as families are pulled apart. Even for service members who return alive and unhurt, the disruption caused by months or years of separation can weaken families.
But war can also impact society in positive ways. It can actually bring about a greater sense of purpose and cohesion for a country. People are able to unite behind a common purpose, as when people shared in the sacrifices (rationing, bond drives) necessary for WWII in the US. It can help a country in the long run in economic terms. This happened in the US, for example when technologies developed for the war transformed the civilian economy after the war.
War is terrible, but not all of its impacts are negative for a society as a whole.