‘We first’ beats ‘me first.’ Ukraine war revives Western alliances.

Amicus certus in re incerta cernitur.

These wise words – best known these days through the English saying “a friend in need is a friend indeed” – were written by the Roman poet Quintus Ennius nearly 2,500 years before Vladimir Putin dramatically intensified Russian missile assaults on Ukraine this week.

But Ennius’ insight has taken on new relevance as America and Western Europe chart their response to the latest escalation in the war.

So far, Mr. Putin’s invasion has had the effect of bringing old friends back together: reviving a...