His reason for veto?
"This resolution is an unnecessary, dangerous attempt to weaken my constitutional authorities, endangering the lives of American citizens and brave service members , both today and in the future," (Quoting his letter to the Senate, per a CNN article).
Yeah let's drag out his usual bullshit rationale chestnut: it 'endangers' US citizen and military. The same rationale he used for the National Emergency re border wall veto. And notice how he ties that 'danger' into the resolution 'weakening his authority'? In other words, give me all the power and you will be 'safe'. Fucking scary!!!!