It's not just threats to America, but threats to our (democratic) allies. My main concern for the past year has been China's actions in the South China Sea, which have become a bigger problem as of late. It's contested area that most people recognize as international waters, but which China, along with the surrounding countries of the Philippines, Vietnam, Malaysia, Taiwan (this is a special case), and Brunei all claim parts of. China is pretty aggressive about it, though, and their claims are some of the most absurd since their country is so far from the area and the other countries have much more history using them as fishing grounds, with Vietnam even establishing a few actual settlements. And an astronomically high amount of international trade runs through these waters. I should describe HOW China is being aggressive, though. They're performing rapid land reclamation projects around these waters on contested islands, and building military bases and airstrips. They declare it their sovereign territory and declare warnings to anybody passing over in the air. Besides this, they've been known to ram fishing boats of nationals of other countries, and they refuse to negotiate any of this except with each country individually so that they have much more pull (as the country that is by far more powerful than any of these individual claimants). It's actually a serious issue, with another country becoming militarized and it possibly pushing towards another bipolar paradigm in international politics. Maybe the US doesn't deserve to be the world superpower, but we're better than the alternative that nobody is really keen on stopping, and the existence and reach of our military is the only thing containing them right now.
Here's the list of military arms sales to Taiwan by the US. That's a lot for a tiny island.