IRL Examples: look at Saudi Arabian borders with Yemen, Oman, and UAE, as well as with Iraq, Jordan, Kuwait, etc. All desert countries have squarish borders, and though it is due in large part to the map drawing techniques of imperialist powers, Saudi Arabia established borders with Yemen and Oman on it's own accord, without simply drawing on a map (if I'm not mistaken there was an actual conflict with Yemen...)
Another note for those people that want their nations to be natural fortresses. The only irl example is Tibet, and unless you pick a highland climate zone I'd be pretty eh about you getting it. Another note about mountains, they *may* act as chokepoints for armies, but they provide EXCELLENT cover for enemy aircraft, helicopters, and special forces sneaking across your border. If you look at the Falklands conflict between UK and Argentina, the Argentinian airforce used small hills as highly effective cover, now imagine what happens when they can just fly over giant mountains before your SAM can track them. Modern war has proven that the air force is the best first strike force around, so choke points are really unnecessary.
