Once all of this is accomplished, we have nothing to fear from enemy troops anyway. A border blockade is not a military solution...we are going to trick the AI into thinking we have created a wall of soldiers. This solution to troops crossing our borders is temporary, and should work until we have the manpower to set up a true blockade.
Quite right. What we are doing here is compensating for the that peculiar "Trespasser" bug that has been written into every version of Civ. For some reason, while the AI civs get quite annoyed when you cross into their territory, their units tend to simply ignore yours, and the only way to stop trespass is to start a war fierce enough to destroy you or them.
In Civ I and Civ II, blocking the path of a trespasser antagonizes the opponent and can cause a war. In Civ III, that is not a serious problem. You can recruit border guards, MPs, Highway Patrol, whatever you care to call them, from your military or civilian units.
As I noted elsewhere, I like to use "explorer" units to serve in the role of scouts, diplomatic envoys, cops, and border guards, because Sid didn't get around to providing those units in the game. Even though, in this case, a border guard would resolve one of the AIs more persistantly unrealistic bugs.
True story from history: In the dark days of 1940, as the British were planning the Dunquerque evacution and Hitler's Panzers were overunning northern France and closing on Paris, Mussolini was debating whether to join his Axis partner in the war. At a diplomatic cocktail party somewhere overseas, a noxious Italian Facist diplomat was expounding on the possibilities of war, mortifying all attending because the ashen-faced French ambassador was among those obliged to listen to his bombast.
"Really, now," the Italian loudly gloated, "If our glorious legions were to march across the border today, what would be there to stop them?"
The Ambassador looked at him sadly, with just a trace of scorn, and gravely replied, "Monsieur forgets the French Customs Service."