BRUSSELS — A plunge in oil prices has sent tremors through the global political and economic order, setting off an abrupt shift in fortunes that has bolstered the interests of the United States and pushed several big oil-exporting nations — particularly those hostile to the West, like Russia, Iran and Venezuela — to the brink of financial crisis.

The nearly 50 percent decline in oil prices since June has had the most conspicuous impact on the Russian economy and President Vladimir V. Putin. The former finance minister Aleksei L. Kudrin, a longtime friend of Mr. Putin’s, warned this week of a “full-blown economic crisis” and called for better relations with Europe and the United States.

But the ripple effects are spreading much more broadly than that. The price plunge may also influence Iran’s deliberations over whether to agree to a deal on its nuclear program with the West; force the oil-rich nations of the Middle East to reassess their role in managing global supply; and give a boost to the economies of the biggest oil-consuming nations, notably the United States and China.

It might even have been a late factor in Cuba’s decision to seal a rapprochement with Washington.

After a precipitous drop, to less than $60 a barrel from around $115 a barrel in June, oil prices settled at a low level this week. Their fall, even if partly reversed, was so sharp and so quick as to unsettle plans and assumptions in many governments. That includes Mr. Putin’s apparent hope that Russia could weather Western sanctions over its intervention in Ukraine without serious economic harm, and Venezuela’s aspirations for continuing the free-spending policies of former President Hugo Chávez.