Naw, you're worrying too much. I've been in a war, and on this planet for almost four times longer than you have, so my viewpoint is a little better. It seems tenser than it really is. This is what countries do, they posture and threaten. What you see on the news and in current world events is what we in the U.S. refer to as the "First World War" still playing out politically. This includes the areas of Turkey, Iraq, Yugoslavia, and North Africa. The "Second World War" is still being played out between Russia and the U.S. in Afghanistan, Iran (which means "Aryan Nation"), and southern/western Asia, and the NATO alliance. The conflicts in eastern Asia predate both of these world wars, and are the least bloody and brutal they have been in generations. The addition of communism there is more a reaction to monarchies and imperialism than an organized plot against the world our propaganda claims it is.
So, there isn't a lot for you to worry about, unless you're in one of these countries. The chances of the U.S., Russia and China trying to invade one of the other, or outright nuclear bomb them is pretty much zero.
If you want to be stressed out and depressed and learn something, read the book "1984" by George Orwell. It was published in 1948, but it is a more accurate depiction of our future than any other novel I've read. In one part, later on in the book, the character Emmanuel Goldstein (it's version of our Osama Bin Laden) explains why these three big countries engage in an unending conflict that never touches their own country.
It's pretty interesting, but really, you're fifteen. Quit worrying about serious stuff and have some fun while you can. If you're in a small country, grow older and move to a bigger country where it's safe. Canada, Australia, U.S., Russia, China, Brazil, the EU. These countries may have their ups and downs, but they're not going anywhere.