Between Sept. 14-20, the Russian armed forces will conduct a major exercise, ZAPAD 2017. Employing forces from various Russian commands, the exercise will test the military's ability to conduct a combined arms offensive. That's exactly the kind of operation Russia would use to invade Europe.

While there's nothing new in the Russians carrying out a military exercise, this situation is different due to the scale and location of the forces involved. Russia claims under 10,000 personnel will be involved but as many as 100,000 Russian soldiers are actually expected to deploy. Their number will include armored and artillery units, and strike fighter aircraft. As interesting, instead of training in Russia, the forces will conduct their exercises in Belarus.

That geographic choice is designed to send a specific message to NATO.

After all, bordering NATO member Poland, Belarus offers ZAPAD forces the means to rapidly strike into the heart of NATO. Joining that threat is the Russian fortress-oblast of Kaliningrad, where reservists have been called up.

Look closely at a map of northern part of eastern Europe, on the shores of the Baltic Sea, and you'll see Kaliningrad — it's a pocket of Russia that doesn't touch Russia, but is instead tucked between Poland and Lithuania. Kaliningrad would be a crucial breaching and supply point for any invasion.

As my map below shows (the red lines exemplify offensive lines), were Russia to invade Poland via Kaliningrad and Belarus, it could encircle Polish forces on that country's eastern front.

The Russians know this and probably believe that they could reach Warsaw before a serious NATO counter-offensive began. While Poland's competent and aggressive military would not yield, it would struggle to withstand a Russian surprise attack by itself.

That said, Poland isn't the only NATO state threatened by ZAPAD; the Baltic states of Estonia, Latvia, and Lithuania face even greater vulnerability. For a start, Putin has long believed those nations are destined to be feudal states of a new Russian empire. And again, the matching of ZAPAD forces in Belarus to Kaliningrad proves the danger.

Invading the Baltics, Putin could use forces in Kaliningrad and Belarus to block Polish/NATO counter-offensives, as launchpads for a two-front invasion of Lithuania, and as distractions for simultaneous invasions of Latvia and Estonia.

Putin might gamble that if his forces seized significant territory in short order, NATO would sue for peace. His intent would be a limited victory that divided NATO between those pushing for military response, and those, like Germany, who would probably assess a counter-offensive as too risky.

Another complication is that NATO's force disposition against a Russian surprise attack remains weak. While NATO would intercept Russian communications in the immediate buildup to an attack, if those orders were sent from Moscow on short notice, there wouldn't be much NATO could do. In addition, as I've explained, NATO's rapid reaction forces in Europe are patently incapable of defeating a Russian invasion. This is a symptom of Europe's pathetic defense investment, and its consequences in rendering that continent one of largely impotent militaries. Putin also loves the fact that many EU populations oppose their government's support for NATO article 5 treaty obligations. Remember, where Germany leads, other NATO states like Italy and Spain might follow.

All of this demands decisive U.S. leadership.

First, the U.S. must lead reliable NATO allies into defensive action. Britain and France should deploy their respective rapid response forces, 16 Air Assault Brigade and 11th Parachute Brigade, into Poland. Simultaneously, Germany and the Netherlands should put their Airborne rapid forces division on short notice standby. The U.S. should deploy the Army's 75th Ranger Regiment's ready reaction battalion to join the 173rd Airborne Brigade in Vicenza, Italy. These units are orientated to disrupt enemy advances and defend critical targets. The 15th Marine Expeditionary Unit (MEU), now at sea, should also be sent to the Mediterranean. Finally, each capable NATO state should send one fighter or bomber wing to the Baltics or Poland.

Alongside these deployments — which should be made public — Trump should again demand that NATO members speed up their plans to spend 2 percent of Gross Domestic Product on defense. He should state that if they do not, the U.S. will have to relocate major military formations in Europe out of states like Germany and Italy and into 2 percent states like Poland and Britain.

The collective impact of these military and political responses would be twofold: They would show Putin that NATO is prepared to fight and, via the 2 percent push, doubling down on a stronger NATO. Yet these responses would also be measured by their scale and defensive tenor. That matters, because Putin would make great propaganda use out of a massive NATO buildup on his borders. His popularity in Russia rests on the notion that he is a tough leader who protects his nation's pride.

Still, there is one huge variable here: the extent — if any — of Russian influence over President Trump. In recent months, Trump has been evidently reluctant to challenge the Russian leader on a number of key issues. And if Putin believes attacking a NATO state won't incur U.S. military reprisals, he will do so. Trump must avoid equivocation on NATO's Article 5 (an attack on one is an attack on all), as he did while meeting Finland's president on Monday.

In the end, while it is unlikely that Russia is about to invade Europe, that threat cannot be discounted. As Putin's bold and ultimately successful incursion into Syria proved, he is willing to take great risks to boost Russia's power and prestige. We must prepare for the worst.