In the last two decades, theres been a an enormous jump in technology available for protecting tanks. You cant just call it armor protection anymore, because what protects a tank these days is not just thick slabs of steel. During the 1980s, composite armor and ERA (Explosive Reactive Armor) were introduced. Composite armor was layers of metal, ceramics and other materials (the exact composition is kept secret). This combination proved much more resistant to all types of tank shells or missile warheads. ERA was blocks of explosives fitted to the outside of a tank. When a shell or missile hit a block of ERA, it exploded, and greatly diminished the penetrating effect. This complicated attempts to describe how thick armor was anymore. Thus began the practice of describing composite armor and ERA in terms if equivalent (to steel) thickness.
Composite armor and ERA made it safer to be inside a tank. For example, during the 1970s, the most common Russian 125mm tank gun shell, the BM-17, could penetrate 250mm of steel at 2,000 meters range. Russia also had a more expensive BM-12 shell (issued in smaller quantities) that could penetrate 350mm. The U.S. M-735 shell could go through 330mm at 2,000 meters. The most modern Russian tank at the time, the T-72, had maximum frontal armor of 350mm. But most of the T-72 was covered with thinner armor, and the most common Russian tank was still the T-55, with max armor of 200mm. The U.S. M-60 tanks had maximum armor of 260mm. Both Russian and American tanks used shaped charge (HEAT) shells, that could go through 400mm of armor.
Today, the composite armor of the U.S. M-1A2 has equivalent armor thickness of 960mm against shells and 1620mm against HEAT. The Russian T-90 (the latest version) has a max of 850mm and 1200mm. But the max penetration for a Russian 125mm shell is 650mm at 2,000 kilometers (the BM-42M). Ukraine says it has a 125mm shell that can do 760mm. The U.S. 120mm M829A3 can go through 960mm, although the most widely used shell is the M829A2, which can only penetrate 750mm.
Thus, while the M-1 has a reputation for being invulnerable, it isnt. But as a practical matter, it has been because, so far, the M-1 has only encountered older tanks. In the two wars with Iraq, M-1s were facing T-72s, which had maximum armor of 430mm and a 125mm gun firing older ammo that had a penetration of 250mm or so. Iran or China, however, equip some of their tanks with ERA, giving them a protection of about 800mm. But more ominously, the 125mm guns of those tanks can use much better ammo, and get penetration of up to 700mm. The M-1 has a lot of thinner armor that can be ripped through with sells like that.
However, the M-1 does, at the moment, have the best combination of combat proven armor protection, very effective shells and well trained crews, in the world. The M-1 is also the most numerous of the tanks of its class (like the German Leopard or British Challenger) and is constantly upgraded. So while the M-1 is neither invulnerable nor invincible, but no foreign tank crew would look forward to going into battle against M-1s.
