The show downstairs was about to begin, but most of the people upstairs didn't care what it was about.
Dumbledore wanted to use this incident to study the relationship between magic and power, so he could adjust the International Confederation of Wizards' structure.
Grindelwald quietly watched how Snape and the Pioneer Organization members, whom fate had hinted at, could disrupt the balance. He wanted to adjust the pace of his plans and take everything into account.
Anton's thoughts were much simpler. He'd only meant to show off a little before leaving. Since he'd run into this, he might as well observe how the wizards' magic changed when they clashed in an incident like this.
And, if his friends ran into trouble, step in personally and get them out.
Voldemort watched impassively from above, waiting to see what kind of force the International Auror Organization—formed to counter his magical army—would unleash, and weighing the strengths and weaknesses of the two approaches they were exploring: organization-wide coordination and an elite force.
And...
Barty Crouch Jr., his capable subordinate whom he'd deliberately kept outside the center of power when he established the Magical Congress.
Back when Voldemort had been in his mad phase, he'd planned to groom Barty Crouch Jr. as his successor. But now, Barty Crouch Jr. was completely unsuited to his new path.
He'd been insane for decades. He knew all too well the kind of damage a lunatic like Barty Crouch Jr. could do to his Magic Empire.
If Barty Crouch Jr. succeeded, he'd receive the Magical Congress's full support behind the scenes, with everyone pushing him toward the position of Chief Warlock of the International Confederation of Wizards.
If Barty Crouch Jr. failed...
Then there'd be no reason for him to return to the Magical Congress, would there?