The money a state borrows against its future taxes — the device that has let governments spend far beyond what they collect, funding wars, infrastructure, and ambitions no single year of revenue could cover. Public debt turns a government's promise to repay into the bedrock of modern finance, which makes the question of whose promises are trusted, and who ultimately pays, one of the central problems of state power.
A government is the strangest borrower in the world. It can tax the very people it owes. It can often print the currency the debt is written in. And it is extremely hard to drag into a court and force to pay. By every ordinary rule of lending, no one should hand such a borrower a penny. Yet states have borrowed more than any household or company in history, and the cheapest borrowers among them have not been the strongest empires but the governments whose promises people found easiest to believe.