Pax Americana is over. The rest of the Western alliance will have to pick up the burden of protecting peace

Open this photo in gallery: European Council President Antonio Costa, Japan’s Prime Minister Shigeru Ishiba, Italian Prime Minister Giorgia Meloni, French President Emmanuel Macron, Canada’s Prime Minister Mark Carney and U.S. President Donald Trump leave after a photo session during the G7 Summit, in Kananaskis, Alberta, on Monday.Suzanne Plunkett/Reuters Michael Ignatieff teaches history at Central European University in Vienna. In five short months as President, Donald Trump has accomplished what the Russians and Chinese never succeeded doing in 80 years: He's splintered the Western alliance. When the remnants of that alliance met at the G7 summit in Canada this week,…