But the NATO alliance is not ready, some military experts say.

If NATO forces ever wanted to transport their tanks, trucks and supplies by rail to reinforce an active eastern frontline, this might be one of their first big logistical challenges.

It's one that slowed down this exercise for almost a full day.

"It's well documented the fact that when you go from Poland into Lithuania, you still have to change to a different gauge because Lithuania, Latvia and Estonia are still on the old Russian gauge rail, as is Ukraine and Georgia. So this adds additional steps."

Ben Hodges commanded the U.S. Army in Europe until 2018. He's been sounding the alarm about the bloc's patchy infrastructure for years.

"What we have learned from Russia's war against Ukraine is we've been reminded actually that war is a test of will and it's a test of logistics. We do not have enough transport capacity or infrastructure that enables the rapid movement of NATO forces across Europe. The Deutsche Bahn, which all of us depend on, specifically DB cargo has enough rail cars to move one and a half armored brigades simultaneously at one time, that's it."

After 25 years of fighting military conflicts abroad, Hodges says NATO suddenly needs to show it can respond to a threat anywhere along its own borders.

The potholed roads of the bloc's eastern members could present another obstacle to rapid deployment.

Motorways account for only 5% of Romania's network - and 28% of its roads are just gravel and dirt.

That's according to the National Statistics Board.

"We've also discovered through exercises over the last few years that the further east you go, it becomes more difficult because the infrastructure is not as robust or redundant. The bridges that can hold a modern Abrams tank or Leopard or British Challenger - not many bridges can sustain that sort of weight."

The EU has allocated 1.6 billion euros - about $1.64 billion - to military mobility projects up to 2027.

Hodges calls that sum "inadequate."

Poland meanwhile, which fears it could be the next target of Russian aggression, is working hard to improve its infrastructure.

A $6 billion high-speed rail track is slated to connect Warsaw with the Baltic capitals by 2030.

Although only $1.2 billion has been allocated so far.

It's part of a much bigger $36 billion effort by Poland to improve civilian and military mobility across central Europe, known as the Solidarity Transport Hub.

Hundreds of miles of railways and expressways and new bridges are planned.

That development at least is welcomed by the former U.S. Army commander.

"I think there's a heightened sense of urgency because of ... We've all been awakened now by the threat of what Russia is doing. And this is not an academic thing, or just a debating topic now, this is real. So I actually am optimistic. I would like to see us having moved a lot faster."