It didn’t take long after Russia attacked Ukraine for French Col. Clement Torrent to get his orders: He had six months to build a base for 1,000 soldiers on NATO’s eastern frontier.

He and about 200 troops from France, Belgium and the Netherlands are now busy leveling a hilltop in the Romanian region of Transylvania. "Our due date is before the first frost,” Torrent, who leads the engineer task force, said from the base near Cincu, about 260 kilometers (162 miles) north of Bucharest by road. "It’s a sign of solidarity. An alliance needs to be tangible.”

As Western powers race to confront the threat posed by Vladimir Putin, Russia’s war on its neighbor has answered a fundamental question that NATO states have been asking for years: whether older members like the U.S., France and Germany will fight for less wealthy ex-communist allies if they’re attacked.