Subtext = under the words.

Many of the readers when the book came out would either have been in the war, lost people in the war, seen the effects of the war on those who came home, or seen the gaps in society left by those who were lost, or any combinations of those experiences. There is a lot that would not have to have been spelled out for that audience as soon as the setting was clearly communicated.

A lot of what he writes is more interesting as a metaphor for the war than as a straight up romance plot.

It's hard to say much without spoiling the book for you. But the great war... it destroyed people. It marked a turning point in how the world thought. Before, we were still traditional enough that men signed up for war, sometimes entire neighbourhoods or factory crews at once thinking they were going to bring honour to their countries and fight with chivalry.

Nobody could have imagined the meat grinder of WWI. It wasn't just that a lot of people died. So many people died and so many returned as physically and mentally mangled wrecks that our entire society was faced with the lie of how we saw ourselves.

This wasn't noble or chivalrous or defensible in any way. WWI was the industrialised slaughter of entire generations. The world would never be the same.

And that disillusionment is what you see in Henry. He displays self-destructive and self-sabotaging behaviour throughout the novel as he fumbles with his love interest as he weighs the merits of the war against the alternatives. A farewell to arms has a very double meaning. The weapons soldiers arm themselves with, or the arms of lovers they say goodbye too.

You haven't read the rest of the book. But Hemingway is a writer who usually tries to do most of his storytelling in the things he intentionally leaves unsaid.

The problem is that the more time passes, the more removed we are from the experiences that fuel his novels. So his meaningful silences become less meaningful as his readers no longer automatically fill in the blanks or grasp the metaphors.