Condoleezza Rice says it is imperative that Vladimir Putin is stopped.
The former US secretary of state has called for a dramatic increase in military assistance for Ukraine to avoid a direct confrontation between the United States and Russia.
In a joint appeal with Robert Gates, who served as defence secretary under Presidents George W. Bush and Barack Obama, that failure to support Kyiv will embolden the Russian despot to attack NATO allies.
The pair backed the joint US and German decision to send armoured fighting vehicles to Ukraine for the first time but has called on other NATO allies to do the same and add longer-range missiles, advanced drones and more surveillance capability.
The duo fear that a lack of significant military development will place pressure on the West to negotiate a ceasefire that would "leave Russian forces in a strong position to resume their invasion whenever they are ready".
In a joint article for The Washington Post newspaper, they said: "The only way to avoid such a scenario is for the United States and its allies to urgently provide Ukraine with a dramatic in military supplies and capability.
"The United States has learnt the hard way - in 1914, 1941 and 2001 - that unprovoked aggression and attacks on the rule of law and the international cannot be ignored.
"It is better to stop him now, before more is demanded of the United States and NATO."
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