The upcoming NATO summit in Warsaw will complete the transatlantic alliance's transformation back to what it was built for: deterring Moscow.

The reason for NATO's return to its old mission is Russia's return to its old ways.  The U.S. Air Force's Gen. Phillip Breedlove, who just recently ended his tour as NATO commander, describes Russia as "resurgent and aggressive."

Consider the record: Vladimir Putin's Russia has lopped off part of Georgia, annexed Crimea and occupied eastern Ukraine, waged cyber-war against the Baltics, threatened Poland with nuclear attack, massed troops on the borders of NATO's newest members, flouted arms treaties, and revived the dangerous Cold War-era practice of conducting mock bombing runs, buzzing Allied warships and testing Allied air defenses.  There were 160 Russian incursions into Baltic airspace in 2015.

Another 2015 data point: Putin unveiled a new military doctrine focused on confronting NATO and pledging the use of Russia's armed forces "to ensure the protection of its citizens outside the Russian Federation."  Given that there are five million Russians in Ukraine and a million in the Baltics – and that Putin has reserved for himself the right to determine when, where, and whether they need to be protected – this is a recipe for something much more complicated than a new cold war.  As if to underscore his intentions, Putin recently reactivated the 1st Guards Tank Army, a large armored force based in western Russia equipped with 500 main battle tanks.

Between 2004 and 2013, Putin – sometimes as prime minister, sometimes as president – increased military spending 108 percent.  Russia's 2015 military outlays were 26 percent larger than in 2014.

In short, even as NATO tried to build bridges to Moscow and avoided building bases in Eastern Europe, even as NATO members slashed defense spending, even as NATO offered partnership to Russia and membership to Eastern Europe, Putin was longing for the bad old days.  As the Brookings Institution's Robert Kagan concludes, "[i]t is the entire post-Cold War settlement of the 1990s that Russia resents and wants to revise."

Perhaps with that goal in mind, Putin boasts, "If I wanted, Russian troops could not only be in Kiev in two days, but in Riga, Vilnius, Tallinn, Warsaw, or Bucharest."

Given Putin's record and rhetoric, it's no surprise that political leaders from NATO's easternmost members – Lithuania, Latvia, Estonia, Slovakia, Bulgaria, Hungary, Romania, Poland, and the Czech Republic – want "a robust, credible and sustainable Allied military presence in our region."  To buttress their request, they cite "the aggressive Russian actions in Ukraine, including the illegal and illegitimate annexation of Crimea ... as well as Russia's military activities in our neighborhood." 

Their worries are well founded.

Within range of Putin's unmarked armies and clever brand of anonymous warfare, Eastern Europe's leaders understand that if Putin follows his Ukraine playbook and covertly violates the sovereignty of the Baltics, he will force NATO to blink or fire back.  Neither alternative leads to a happy outcome.  The former means that NATO is neutralized.  The latter means war.

The surest way to prevent those dire scenarios is to answer Eastern Europe's SOS and base permanent NATO assets where they are most needed: on the territory of NATO's most at risk members.  That's what the alliance did during the Cold War, and it kept the peace – as it will today.

Indeed, British general Richard Shirreff, former deputy commander of NATO, says the best way to prevent war in Europe is "to maintain troops permanently in the Baltic states." 

The good news is that Putin now faces an alliance renewed in purpose.

Sixteen NATO members increased defense spending in 2015.  European defense spending is up 8.3 percent in 2016.  Germany, for the first time in 25 years, will expand its military endstrength by 14,300 personnel.  Washington has quadrupled U.S. military spending earmarked for Europe – from $789 million to $3.4 billion.  A U.S. defense official says NATO is "moving from assurance to deterrence."

Toward that end, after years of waning commitment, the U.S. Army is increasing its deterrent strength in Europe by permanently basing three fully manned brigades in Europe.  NATO is hammering out plans to deploy battalions in Poland, Latvia, Estonia, and Lithuania to deter Putin from the sort of ambiguous, anonymous warfare he has waged in Ukraine.  And importantly, Breedlove's successor, the U.S. Army's Gen. Curtis Scaparrotti, comes to NATO from the 38th Parallel, where U.S. troops serve as a 24/7 deterrent against North Korean invasion.

The bad news is that NATO members have been hacking away at NATO's deterrent strength, which explains NATO's urgent call that each member invest at least 2 percent of GDP in defense.  Only five of NATO's 28 members meet that standard today.

Years of underfunding have led to "alarming deficiencies in the state of NATO preparedness," according to the British government.

For example, post-recession austerity measures have reduced the Royal Navy from 89 ships to 65.  Britain's combat aircraft fleet has shrunk from 189 warplanes to 149; the Joint Helicopter Command had 257 aircraft in 2008 but has just 164 today.

Only 42 of Germany's top-of-the-line 109 Eurofighters are in flying condition.  At the height of the Cold War, West Germany had 2,125 Leopard II tanks.  Today, Germany has just 225.

The French military eliminated 8,000 personnel in 2014, 19 warships between 2009 and 2012, and 30 percent of its air fleet between 2008 and 2013.

The U.S. Army has around 26,000 troops in Europe today, down from 40,000 in 2012, down from 300,000 during the Cold War.  Thus, the U.S. Army's Gen. Ben Hodges is trying "to make 30,000 look and feel like 300,000."

Moreover, Washington's response to Russian aggression – and to Allied anxieties – reflects the problematic "lead from behind" approach that characterized most of President Obama's foreign policy.  Thus, rather than a robust commitment of a brigade or more in the Baltics and Poland, the Obama administration is offering a battalion to NATO's tripwire force in Eastern Europe – and apparently reneged on earlier pledges of two U.S. battalions.

Despite Washington's halfhearted reaction, what NATO's easternmost members are requesting is feasible, compatible with NATO's core mission, and militarily credible.

First, NATO has about 3.3 million men under arms and accounts for 60 percent of world military spending.  In other words, the alliance can do this – but only with a renewed commitment to its enduring mission of deterrence.

Each NATO member should lift its defense budget to the 2-percent-of-GDP standard by a date certain, each member should invest in a way that serves the needs of the alliance, and Washington should lead from the front by reversing sequestration's devastating cuts.

Second, basing a deterrent force in Eastern Europe is in line with NATO's core mission of deterring war.

For the United States, NATO diminishes the likelihood of another European conflict triggering another great-power war.  For NATO's other members, NATO is a security guarantee backed by the United States.  Without that guarantee, there is no security, as history has a way of reminding those on the outside looking in, from Cold War Hungary to post-Cold War Ukraine.

Indeed, the reason Poland wants U.S. troops on Polish soil is the same reason U.S. troops were based in West Berlin during the Cold War: American troops send an unmistakable message that crossing this line means you are going to war against the United States – no question marks, ambiguity, or doubts about the consequences.

Third, permanent bases in the Balts and Poland will signal Moscow that NATO is serious about defending Eastern Europe.  The goal is to prevent what Churchill called "temptations to a trial of strength" by making it clear to Putin that NATO's security guarantee is as valid for NATO's youngest members as it is for NATO's oldest members.

Alan W. Dowd is a senior fellow with the Sagamore Institute.