Is investing in a Single Country a smart move?

I've been toying with the idea of putting some money into a single country's market rather than diversifying across several. I've heard both sides of the argument. Some say focusing on one country can lead to higher returns if you pick the right nation, especially one that's on the up-and-up economically. But then there’s the risk factor. What if that country's market tanks or political issues arise? Have any of you taken this plunge before? What was your experience? Did it pay off, or did you find yourself regretting not having a more diversified approach? I’d love to hear your thoughts!