I need a subject for a story.

Maybe about a family back in those times and with a father who is always gone.

Well - if you mean the American Civil War, here's some possible subjects.

The war was run extraordinarily badly. For instance, more Union soldiers were dying of bad management at their camps - disease in particular - than from enemy fire. In one camp, the soldiers painted "Cafe de Worms" on their mess wagon, because they were reduced to grubbing for earthworms and baking/boiling/etc them for food. Meanwhile people who were rich enough to send a 'substitute' were able to evade conscription. This lead (in one case) to rioting in New York amongst the poorer people who couldn't afford to send a substitute. A soldier from one such family could send a letter home (about "Cafe de Worms" and all the rest). That could be the story - told in his letter home.

How the soldier feels about this war depends on what you want to say about the war. Say it was justified because (in the end) it became the catalyst for ending negro slavery in USA. So perhaps this is a story about "fighting a war for something bigger than myself NOT FORGETTING that those running this war have done a terrible job with the logistics."

Another option for a Civil War US story is to make an escaping slave your main character. He might hear about the war, overhearing his 'owners' talk about it on the porch and decide "I'm going to run away and join the Union army!" And perhaps the emancipee comes to realise that the North is not perfect either. For instance, even Abraham Lincoln was reluctant to let negroes join the Union armed forces. Race prejudice was strong in the North. BUT despite this, the escapee still says "It's better being here in freedom than still in the South in slavery." Again - a story about something bigger than oneself.

Something a teacher taught me about story writing was: Choose your premise. A premise is (in old fashioned language) the "moral" of your story. What is it you want to say? "War is hell," "War can achieve useful outcomes, but what a mess it makes in the process!", "There's nothing new about wartime corruption/mismanagement" etc. When you've decided on your premise, the rest of your story starts to 'fall into place'. Though sometimes starting with a story you want to tell works best - because when you want to tell a particular story, it's usually because it MEANS SOMETHING TO YOU and that's the essence of a premise.