Final answer:
The US historically might respond to a neighbor refusing to pay debts by exerting economic and military pressure. During Taft's presidency, he paid off Central American debts to Europe and used military force in cases of resistance, like in Nicaragua.
Step-by-step explanation:
When a neighboring nation refused to pay its debts to US allies, the historical response of the US could vary based upon different eras and contexts. However, based on the information provided, one can infer the US might respond by exerting economic and military pressure to protect its interests and those of its allies. During President William Taft's administration, for instance, a similar situation arose with Central American countries when they resisted US financial help to pay European debts. Taft moved to pay off these debts with US dollars, making the countries indebted to the United States. When resistance was met, such as in Nicaragua, Taft responded with military force, by sending a warship with marines to the region. Moreover, the US has historically provided military and economic assistance to allies when requested, in alignment with treaty commitments.