With continued worry that Russia's bloody invasion of Ukraine could spill over into neighboring countries, the Pentagon is putting in place the military infrastructure needed to help run a war in Europe and meet President Joe Biden's pledge to defend "every inch" of NATO territory, according to several defense officials.

The Pentagon has deployed thousands of additional troops to Europe, including 500 more US troops announced Monday. The US is sending defensive weapons like Patriot missiles to Poland and considering moving additional assets to protect other allies like Romania and the Baltic nations of Latvia, Lithuania and Estonia from any Russian moves on their borders, officials say.

A spokesman for US European Command told CNN on Tuesday evening that the United States is sending two Patriot missile batteries to Poland as a “defensive deployment” to counter any potential threat to US and NATO allies.

The Pentagon estimates that as many as half of the 670 missiles Russia has fired against Ukraine have been launched from inside that country, raising concerns that it can more easily reach NATO countries if it chooses to lash out or expand the scope of the war.

In all, the US has repositioned nearly 15,000 military personnel in reaction to Russian aggression. The total number of US troops in Europe is now expected to reach 100,000. The US effort comes along with a dramatically stepped-up NATO deterrence, as any US military operation would almost certainly be under a NATO umbrella.

“We have 130 jets at high alert. Over 200 ships from the high north to the Mediterranean, and thousands of additional troops in the region,” NATO Secretary General Jens Stoltenberg said Tuesday while traveling in Latvia.

The latest deployment includes an “air support” operations center heading to Poland and Romania, as well as refueling and maintenance specialists to Germany to bolster US and NATO air operations over the region. The Army's Fifth Corps also is deploying 300 headquarters troops to Poland and Germany to provide command and control for US ground forces.

The US Navy briefly positioned a carrier in the northern Aegean Sea this week that could be used in a contingency for flight operations should tensions escalate.

Allthe troop movements had long been expected, a senior defense official insisted. But there is no question it's all about protecting against any possible Russian incursion into NATO member nations.

While the US is adamant about its intent to avoid a direct conflict, the Pentagon's deployments are aimed at putting a military structure in place both to deter Russia from widening the scope of its war and to prepare against a deliberate or accidental spillover of the fighting into NATO territory.

“One of the things that we've talked about is how important protecting NATO territory is, and that includes NATO airspace,” a defense official told CNN.

In the air, dozens of US aircraft — including advanced F-35s fighter jets and intelligence and reconnaissance planes — are conducting daily patrols on both deterrence and intelligence collection missions, while staying outside Ukrainian airspace and out of the reach of Russian anti-aircraft weapons.

Protecting against a wider war

Biden and top US officials have repeatedly stressed that US troops will not fight in Ukraine, which is not a member of NATO. At the same time, however, senior US officials have continued to reiterate that the US commitment to defend fellow NATO countries in the event of an attack is “sacrosanct.”

NATO's Article 5 doctrine says that any attack on one NATO country is an attack on all — meaning the US and other European allies would be obligated to come to the defense of any NATO member attacked by Russia.

The moves are a recognition that the US military has to be prepared for the contingencies that the biggest land war in Europe since World War II may not remain confined to Ukraine's borders.

Part of the concern among US officials is that Russian President Vladimir Putin could decide to attack NATO nations, especially if he feels backed into a corner. Last week, the Kremlin put forward a list of “de facto unfriendly countries,” including the US.

Since Russia invaded Ukraine on February 24, the US and its allies have imposed a series of punishing financial sanctions on Russia, which Putin has since likened to a “declaration of war.” Russia has also issued increasingly threatening rhetoric about countries that send weapons to Ukraine.

On Tuesday, the Pentagon dismissed a plan floated by the Polish government to deploy its MiG-29 fighter jets to a US base in Germany so they could be delivered to Ukraine. Pentagon press secretary John Kirby said in a statement that the proposal was not “tenable,” in part because the prospect of fighter jets “departing from a U.S./NATO base in Germany to fly into airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance.”

Retired Lt. Gen. Mark Hertling, a national security and military analyst for CNN, said the Biden administration's reinforcement of NATO is important both to assuage the fears of NATO countries that had worried in the past about the US military's commitment to Europe and to send a clear message of deterrence to Putin.

“Putin went into this whole thing with Ukraine thinking the US wouldn't do anything, and I think he's very surprised at what he's seeing right now,” Hertling said. “I don't think Putin is stupid enough to go one on one against NATO … but if he does, he will be outgunned and outmanned, other than nuclear weapons.”

An umbrella of air defense

As anxiety mounts in Eastern Europe that the Russians could launch a potential attack against NATO's eastern flank, the US is increasing its defensive missile capabilities there, according to a US official familiar with current administration thinking.

“The US is considering a range of capabilities,” the official said. Along with the move of Patriot missile batteries to Poland, potential shipments could also include the THAAD anti-missile systems, which have successfully been used in the Middle East to shoot down incoming ballistic missiles.

There is no specific plan yet, but the idea centers on the concern that Russian missiles or aircraft might attack targets inside NATO territory at some point if Putin decides the support of those nations for Ukraine threatens his plans there. And because of the close proximity of the airspace, there is also concern that air defense be available if Russian aircraft inadvertently stray into NATO airspace.

The establishment of a so-called US-Russia deconfliction line was in part to address those worries.

But there is a sense an umbrella of air defense is needed, not just a means of communication.

Because of the defensive nature of these systems, the US will emphasize that Russia should have no concerns.

US naval preparations 

US naval forces are also scoping out how they might respond to Russian aggression. The aircraft carrier Harry S Truman has been in the northern Aegean Sea this week in part to ensure it can conduct flight operations from there if tensions escalate in the region, according to a US official familiar with the carrier's current task.

The official did not disclose the exact location of the ship but noted it's not routine to operate in the Aegean's northernmost waters. The area is relatively confined for carrier operations due to the number of small islands and heavy commercial maritime traffic.

“In the event of further escalation,” the Navy could be told to project more fighter jet air patrols into the Black Sea, the official said. “If Russia sought to run the Turkish strait and Turkey asked for NATO support,”  then carrier aircraft could be used for combat air patrols over the Black Sea and Strait of Dardanelles.

Turkey administers passage permission to transit into the Black Sea for warships and there is concern about Russia's future plans there, so keeping that waterway open would be part of any potential US and NATO effort if Turkey requests help.