Ukraine's military isn't the only one that's been gearing up for its counter-offensive.
Russian combat engineers have been busy building fortified networks layered with treacherous obstacles that Ukraine's forces will have to shatter rapidly as they likely come under attack by artillery, drones, and missiles.
These defenses, even if they rely on age-old strategies, appear formidable: Lines of pyramid-like obstacles known as dragon's teeth and designed to block even tracked vehicles like tanks. Ditches as wide as 20 feet to trap armored vehicles. A trench line with fortified firing positions. Mine fields and razor wire.
Add to this Russia's firepower from artillery and drones, and any moment of delay in defeating these obstacles could be murderous - especially without the airpower Ukraine needs but largely lacks that could even the odds for ground attacks.
Satellite images and researchers have shed light on what lies in the deadly zone beyond the line of contact. Russia's defenses generally have two to three lines of trenches that run as deep as 18 miles, with the rear-most line offering heavy fortifications for command posts and fallback positions, according to a report by Jack Watling and Nick Reynolds of UK's Royal United Services Institute.
And Brady Africk, of the American Enterprise Institute, has documented heavy defenses that blot across from the Dnipro reservoir (which is losing its water due to a destroyed dam) to Russian-held areas in Ukraine's industrial east.
In an arms package announced Tuesday, the US announced it is sending more "demolition munitions for obstacle clearing" on top of the C-4 plastic explosive and unspecified "demolition equipment" it has already sent. These provisions could help Ukraine's sappers and heavy mine-clearing vehicles break through, creating the opening for the assault force to pour through.
To be sure, the defenses are surmountable. Nazi Germany's defenses, like the Siegfried Line built to defend its west, also featured dragon's teeth, machine-gun pillboxes, razor wire and mines.
And there's the possibility that Ukraine might be able to avoid these defenses by avoiding the roads and areas they block. As Insider's Jake Epstein has reported, Russian positions may be vulnerable to attack from assaults that cross open terrain and steer clear of roads.