If you were to name the very top tier of the wizarding world, it would obviously be Dumbledore, Grindelwald, Anton, and Voldemort.
Setting aside the rising stars Snape, Fiennes, and the people at Cabin, or Lockhart, who might catch up with them one day.
Of those four at the very top, only two still devoted their energy to magic.
Anton and Voldemort.
One focused on building Magical Theory and researching magic's applications in everyday life, while the other was devoted to using magic as a means to advance his ambitions.
That was obviously terrifying.
Dumbledore was indeed powerful, but the magic he excelled at—Transfiguration, Legilimency, Alchemy, and so on—was rarely used to support his ambitions.
Voldemort, on the other hand, had managed to make magic and his ambitions complement each other.
Who could stand against him?
No one could deny the Dark Lord's terrifying breadth of knowledge, whether it was the magical intelligence and spellcasting ability he had displayed as a student—far beyond his peers and even most people of his era—or the results of the Magic Corps, which had only recently come to light.
Note: this wasn't a wizard army. It was a magic army.
Ordinary wizards, Squibs, Goblins, House-elves, Giants, Centaurs, Fire Dragons...
Everything capable of fighting had been incorporated into the Magic Corps.
And that was without even mentioning the help Voldemort had received from the Muggles after he began accepting them.
Muggles were good at more than just using tools like satellite networks. Many of them possessed kinds of intelligence that wizards could scarcely match.
For example, the military commanders and group-combat training officers in the Magical Congress, who were skilled at warfare.
Though they had only been transformed into the most basic kind of Squib, barely able to cast one or two spells, they played