Manifest Destiny goes well beyond the conquest of what is now US territory. It has been and still remains the imperative of US policy around the world. Manifest Destiny is why the North fought the Civil War with the South. To abridge the Union was to deny God's mandate. It is why the US went to Viet Nam, Korea, Germany twice, Iraq, and is now waging a global war. We must maintain globl supremacy, it has been ordained by God, by reason, by the fates.
Folks not from the US would do well to understand that Manifest Destiny is a directive from our God, our ancestors, our fate and not a term of historical trivia.
^^ A bit strident perhaps but none the less this sense of absolute righteousness is engrained in our culture and our body politic.
Anyway, a Manifest Destiny t-shirt is just a more polite form of "America, F#$@ Yeah!"