grammar - What is this との?

For full context: http://www3.nhk.or.jp/news/easy/k10011218521000/k10011218521000.html

The sentence in question:

「中国やロシアから旅行に来る人がビザを取りやすくなったり、韓国や香港などとの間を飛ぶ飛行機が増えたりしたためです。」

"This is because it became easier for travelers from china and russia to get visa and because the number of aeroplanes flying the space between korea, hongkong and so on has increased."

Why is it 韓国や香港などとの間? Why is this と there? I would've expected a mere の.

Answer

Let us first get the basics down by looking at a simpler phrase. How do you usually say something like "the border between France and Germany"? Native speakers say:

「フランスとドイツの国境{こっきょう}」 informally

and

「フランスとドイツとの国境」 formally. ← Needs two と's!

Thus, the formula here is:

"A + と + B + との + Noun"

We do use that second 「と」 when speaking/writing more formally, and I know from experience that quite a few Japanese-learners are not aware of this. Since you seem to be reading the news, you would naturally encounter more formal phrases and expressions.

Now, the actual phrase in question.

「韓国{かんこく}や香港{ほんこん}などとの間{あいだ}」

From the context, this means "between (South Korea, Hong Kong and other countries) and Japan". Agreed? This piece of news is about how many tourists Japan receives from other countries.

In other words, this part of the sentence is not talking about what happens between South Korea and Hong Kong.

Remember the double-と rule for formal speech?

「韓国や香港などとの間」

means none other than:

「韓国や香港などと日本との間」

Or, for more visual appeal, it is saying:

「(Some countries) + と + 日本 + と + の + 間」

Hope you enjoyed my "two - と - rial"!