The deployment by the United States of an armored brigade in Poland on Russia's border is quite reasonably seen by Russia as a threat and a provocation. One need only look at historical maps of the invasions of Russia from Napoleon to Hitler to understand why.
Moreover, an armored brigade is essentially an offensive formation. Spearheaded by tanks, its purpose is to bring "mass" to a battlefield, as military geeks call concentrated firepower. It is designed for attacking, even if it is just a counter-attack that takes place in the framework of a retreat.
An armored brigade's punch comes from tanks, backed up by a large number of armored fighting vehicles and personnel carriers.
There is a very large infrastructure you need for this kind of force, in logistics (supplies), communications and other armored vehicles that are not primarily for fighting, but, for example, for evacuating the wounded, hauling a 70 ton Abrams tank to where it can be repaired, fording rivers, clearing obstacles and so on.
This sort of infrastructure can be easily expanded to service 2-3 brigades, an armored division), but, in the case of Poland, it is said explicitly that facilities will be developed to host much larger forces in a "crisis." Russia has to assume what is being developed is the staging area at least for an armored corps, as the United States and Germany have more than enough resources in the neighborhood to quickly put one together.
In (very schematic) military language, a "corps" nominally consists of three divisions, each composed of three brigades. You're talking about 30,000-50,000 troops with hundreds of tanks and attack helicopters, thousands of armored vehicles of, a huge supply chain, in short, a very credible component of an invasion force.
Moreover, U.S. brigades are very heavy compared to those of other countries. They are almost as large and fully as capable as what military doctrine in other countries designate as a "division." These designations are not arbitrary but describe a set of capabilities, especially the ability of a unit to fight as a coherent unit bringing together a variety of resources, like armor, artillery, tactical air support, logistics, command and control, etc. A U.S. brigade (especially if re-enforced with a few other specialized units as part of what the Pentagon now calls a task force or combat team) will look to the Russian side like a division, therefore a U.S. division will likely be seen as an armored corps, and an armored corps like a full army.
Russia cannot ignore that sort of NATO capability on the other side of their frontier, because if fast-moving enemy forces break out into your rear, you're fucked. That happened to the Soviet Union in 1941, and it came within a hair's breadth of losing the war, but was saved by the onset of winter (just as Russia had been when invaded by Napoleon).
What is the justification for this Western provocation? That everyone is in a panic after Russia's takeover of the Crimean peninsula three years ago, and therefore need to be reassured.
What people are not being told is that historically, Crimea had never been part of the Ukraine. The big majority of the population is Russian. The biggest city in the Peninsula, Sevastopol, was and is the headquarters of the Russian Black Sea fleet and to all intents and purposes, was in the Soviet Union and remained after Ukraine independence under the control of Russian armed forces, while under Ukrainian civilian administration and nominal sovereignty.
Crimea became associated with the Ukraine when Nikita Khruschev transfered it from Russian to Ukrainian administrative jurisdiction in the mid-1950s. The idea was to promote the Russification of the Ukraine and undermine Ukrainian nationalism. It made little difference otherwise, as Crimea, along with the Ukraine as a whole, had been and continued to be part of the Soviet Union.
When the Soviet Union fell apart in the early 1990s,the status of Crimea remained up in the air. At one point the local government there declared independence but could not make it stick. Sevastopol remained under Russian economic and military control because of the Black Sea Fleet. Eventually it was settled by an agreement that left Russia in direct, exclusive control of most of the harbor and parts of the city; authorized the presence of many thousands of Russian troops on the peninsula, and recognized Crimea as a an autonomous Republic within the Ukraine, a status no other part of that country enjoyed.
In 2014 the very corrupt president of the Ukraine was driven from power, with right-wing nationalists taking advantage of popular unrest.to seize control through a parliamentary coup. The very first law that the new authorities adopted in effect banned Russian as a recognized, official language, which was seen as a declaration of intent to drive out Russian interests in the country.
It was very satisfying, especially to ultrarightists, but handed Crimea on a platter to Russia. The local governments of Crimea and Sevastopol started organizing a referendum. Putin made clear that should the peninsula decide to "return" to Russia, of course their status and rights as Russian citizens would be automatically recognized and instantly in effect. (Which would triple pensions and raise the wages of government employees, among other things). And at the same time, armed detachments without insignias quickly took over government offices, posted guards at banks and post offices, and basically taken control without a shot being fired, or even a shouting match.
These were described in the U.S. as units of Russian soldiers and local militias, which was tremendously confusing, because, when did the Russians invade? And where did these militias come from? It had only been a few days since the coup in Kiev.
The truth is that the Russian troops had been there all along, some 15,000-20,000, all perfectly legal, open and above-board in keeping with the agreements between Moscow and Kiev. The Crimean militias were the local cops and military folks.
Obviously Putin had been extremely helpful and encouraging in Crimea's "return" to Russia, and would have brooked no opposition But the Russian population of the Ukraine was happy to accept it. And for Moscow, it was essentially a defensive move in reaction to the central Ukrainian government in Kiev having fallen into ultra-rightist anti-Russian hands.
For the Russians, it should be obvious to anyone that it acted to defend strategic geographic interests in a distinct territory that was already its military protectorate. And NATO claims about countering possible Russian "aggression" elsewhere are bunk.
But the NATO build-up does give rise to Russian strategic military considerations. Russia is going to build up its own infrastructure and forces on the other side: from a conventional military point of view, it has no choice. This will "confirm" Russia's expansionist intentions, when in reality they will be defensive.
How can anyone tell that it is NATO and the Americans that are driving this? Because the American deployment is sold as a move to "reassure" and calm the nerves of Russia's neighbors, in other words, Russia has done absolutely nothing in that area that can be presented as aggressive or threatening.