It'd be at the barrel of a gun. Better to have a casus belli to get people willingly volunteering. Without that, you got a whole bunch of people who at best halfheartedly join to avoid prison, like what's going down in Russia. Putin didn't give his people something to die for, unlike in WW2 when the Nazis were coming in with the express intent of wiping every Slav from the face of the earth.
America's playbook has always been to have something for people to willingly for. If someone's momma comes around screaming why her son came back in a box, the Feds gotta be able to be like "Hey, they started it. Your son died in defense of this nation

".
French and Indian War - French and Indian incursions on setters on the frontier
Revolution - Intolerable Acts
1812 - Impressing of sailors/ Native incursions on settlers in the frontier (really American expansion on Natives masked in manifest destiny)
Mexican American War - Mexican incursions on settlers (again American expansion on Mexican claim)
Civil War - Fort Sumter attack
Spanish American - USS Maine
WW1 - Zimmerman Telegram
WW2 - Pearl Harbor
Vietnam - Gulf of Tonkin
Persian Gulf - Kuwaiti Independence
Gulf War 2/Afghanistan - WMDs/9/11
The government's gotta appeal to Zoomers, so that why you saw a whole bunch of Ukraine is so progressive propaganda in the media (Ukraine legalizing gay marriage and shyt like that)