The pre-Ukraine consensus that force should not be used to change international borders was the product, not of abstract rules, but of the following:
- Lingering memories of the destruction caused by World War II, and a general desire not to repeat the experience;
- A principled rejection of colonialism; and
- American military power as a backstop.
All three of these things have waned over time to varying degrees; hence, the invasion of Ukraine. One has to hope that the images of the destruction in Ukraine, and the failures of the Russian military in the face of Ukrainian courage and NATO weapons, will revive them.