Let's assume, for the moment, that we are all citizens of fair Peacelandia, a land of Kumbaya and gentility. Naturally, no such country exists on this Earth, but assume for a moment it does, but it also has an Army, because not entirely trusting of The Goodness of Mankind. We share a long border with Conquestia, who are evil mean, nasty and not nice. Lately, while professing peaceful intentions and in no way wishing to break the peace, they have dispatched six or seven of their best divisions, a healthy chunk of their army to our border. Should we be concerned?
The short answer is Hell Yes!
The reason is very simple, rooted in history, and has its core in the nature of military operations. In peacetime, except for periodic exercises, where soldiers go out and practice what they do on a large scale, soldiers stay in barracks. Barracks on, or around a base, are where soldiers live. It's their normal sleeping area, drinking area and training area. Bases tend to be located near their training areas, which means that except at the end of a major exercise soldiers are at their very best when there or coming off a major military exercise.
So now they've been ordered to move to the border of Peacelandia. Presumably Conquestia's General Staff (or whatever they call it) has prepared for this with extensive planning of the routes and means by which the soldiers will move to the border. Before World War I the preferred method to move troops in quantity was by railroad. In fact, it still is. Back then military officers were all experts at railroad timetables. But once at the border, becasue Peacelandia can be expected to tear up any connecting tracks, assuming they use the same railroad guage. This means instead of riding forward, they have to move on their own. Today, for the U.S. Army that means they move by truck, helicopter or armored vehicle. But for most of the world's armies, they move the same way they did in the First World War, on the Mark One foot. They march. So everything changes at the border.
Second except for setting up camp, laying out communication wires, digging foxholes, etc, Conquestia's army likely isn't doing much. For a few days it's just like another exercise, but after that they'll be eating from field kitchens, sleeping on cots (if they're lucky) and playing a lot of cards. If they have armored vehicles, particularly tracked vehicles, they'll load off their transporters, and start driving. At that point their vehicles will also start to break. The maintenance workload will go up. One simple rule is: the longer in the field, the more maintenance will be required. They'll patrol of course, and may even slide forward to their expected jumping-off positions, but if they do that too much there's a chance that Peaceland's scouts will notice and divine their intentions. They'll also have to maintain radio silence in order to keep from giving away their positions. Ideally, they'll base somewhere where it will be hard for Peaceland's observers to find. That worked very well for the Germans twice in World War II, when they hid in the Ardennes before attacking using the 'sickle cut' strategy that collapsed the French Army in 1940 and later before the Battle of the Bulge. That isn't always possible of course, and these days we have infrared sensors which can sease their heat on the ground, but they aren't perfect and if you're massing on the border maybe you want to be seen.
That's certainly the case if your objective is limited. Imagine Conquestia had beef over Peacelandia's fishermen pushing too far into a sea they both share. If that's Conquestia's true political goal, the buildup will follow a long series of forcussed complaints and political posturing before finally the troops are moved foward. And if that is the case,it's likely a diplomatic solution will soon follow. Of course, we're taking about Conquestia here, and they are vicous no-good bastards who've eyed Peacelandia's verdant plains for decades now. And they aren't really talking about fish.
It's also possible that Conquestia hasn't really made up its mind, and hopes simple bluster will accomplish its objectives, maybe to pacify the more nationalistic members of their populace. But this leads to a second problem. When your troops are at the border, they aren't doing much until told to move. They aren't exercising, they're playing cards. Conquestia's officers will do all they can to keep their troops busy, but the trutth is there are 24 hours in a day, and they won't be able to occupy them as fully as the would on an exercise. If they exercise, they'll wear out their equipment. in fact their equipment degrades every day in the filed. So the longer you stay in a fixed postion, the more your combat power will degrade This is a truism. Conquestia's Generals can be expected to remind their political masters of this. It may not be becasue they're eager for war, soldiers as a whole are much less eager for war than politicians who as a class rarely have to fight. But the soldiers know that if sit too long doing nothing at the border, their army loses combat power. it's why in World War I when the Russians announced they were mobilizing their army Germany went ahead and declared war. They figured that if the Russians were going to war anyway, so their was little point in sitting around and waiting for it. In fact there are good reasons not to wait, like maintaining the initiative.
The problems for the good people of Peacelandia are different. Maybe Conquestia isn't planning to invade. It's possible, right? Peacelandia may hope that the other nations of the world will put pressure on Conqestia to pull back and give up this mad aggression. Maybe Conquestia simply seeks attention; North Korea plays that card all the time. Seeking international support. seems to be Ukraine's strategy today. It makes sense. Ukraine is not a pygmy, but Russia is a power only at military force and the production of fossil fuels. That and cyberespionasge. The problem is that Russia inhertied much of the former Soviet Union's nuclear arsenal. Ukraine inherited some too, but they Did The Right Thing and gave their up hoping to set a good example for the rest of humanity. Given that Russia has conquered Georgia, Chcechnya and the Crimean Peninsula, a part of Ukraine, my guess is the Ukrainians very much regret that decision today. I doubt if the Russians would have touched Crimea or be massing troops if the Ukraine had nukes. But Russian nukes also complicate the problem of those who might help them. After all, is Ukraine really worth risking World War III?
Still, Peacelandia must come up with a valid strategy for defending their country. The smart thing to do is to keep their main army back from the border, out of the range of Conquestia's artillery. Use light forces and anti-tank teams to slow the bastards if they invade, identify their objectives and if passible, canalize them into a place whee Peacelandia's generals prefer to fight. Counterattack with your full forces and hurt them. But that means you must abandon your own citizens who live at the border. The good people of Bordertown are brother citizens, and it is unthinkable to abandon them! Many general officers would argue for this, recognizing the military advantages of staying back and fighting a mobile defense.. But our President-for-Life almost always feels differently. Peacelandian soil is holy soil! it must be defended to the last man! Likely he is far from the only Peacelandian to feel that way. Fighting at the border is a lot easier politically.
It really is easy to consider a mobile defence because it's the best defence if your soldiers are well-trained. But consider South Korea and Seoul, its capital and largest city is right on the Demilatarized Zone. An awful lot of South Koreans live in Seoul. Ready to sacrifice them?
So really, when a powerful enemy is massing at your borders, they likely aren't just there for the scenery.