The initial Western response to Russian aggression in Crimea has emphasized diplomacy and economic measures, and a temporary show of military support to North Atlantic Treaty Organization (NATO) allies bordering on the former Soviet Union. With Russian forces now occupying some positions in the southern mainland and massing on Ukraine’s eastern border, stronger -- and permanent -- US and NATO defense measures are required to deter Russian aggression and raise its costs. There will be no return to a Cold War, but Russia is now a military adversary, even as it remains a partner in other pursuits such as the International Space Station.

Without robust military steps, the West cannot achieve the two ambitious goals the White House defined on March 6 -- the pull-back of Russia’s military forces to their bases, and the restoration of Ukraine’s territorial integrity. Further, a weak response would risk escalation of Russian aggression against its neighbors, and undermine the confidence of US allies and friends in America’s policy of extended deterrence. One or more of them might conclude that they have to pursue their own nuclear weapons programs.

Earlier this month, the Lithuanian ambassador to the US voiced concern about the viability of NATO's Article 5 guarantee, an attack on one means an attack on all. A credible military response to Russian aggression will be important to quelling such doubts.

In December 1994 in return for Ukraine’s agreeing to give up remaining Soviet-era nuclear weapons, the United States, Russia, and the United Kingdom signed the Budapest Memorandum, pledging to respect the borders of Ukraine and abstain from the use or threat of force against it. In addition, nuclear powers France and China provided unilateral security guarantees. In 2009, Presidents Barack Obama and Dmitry Medvedev reaffirmed the Budapest pledge. The Western guarantors must now make good on their solemn commitment.

The West has taken temporary but important military steps, including added combat aircraft support to a NATO Baltic air policing mission and the dispatch of sophisticated airborne warning and control aircraft to Poland and Romania. The AWACs deployment will improve air situational awareness over Ukraine and increase some warning times.

An effective Western military response to Russia’s aggression, however, must be more forceful and have lasting effect.

One place to begin is by revisiting the wishful 2010 NATO Strategic Concept. It expresses a desire to pursue a "true strategic partnership" with Russia, while making no mention of its posing adversarial risks. This thinking appeared to ignore Russia's cyber attack on Estonia in 2007 and invasion of Georgia in 2008; the Kremlin’s new strategic doctrine in 2010 that falsely claims one of the "main external threats of war" is NATO's expansion eastward to Russia's border; and Moscow's persistent but inaccurate characterization of NATO theater missile defenses as threatening Russia's nuclear forces.

To reduce risks to Ukraine, Georgia, and other Black Sea littoral neighbors of Russia, France should announce that it will not deliver the two modern, large helicopter assault Mistral-class ships it is building for Russia as long as its troops occupy neighboring territory. One ship is planned for delivery to the Russian Black Sea fleet in 2016.

The West should provide more and immediate defense aid to Ukraine and Georgia. Early priorities ought to be intelligence sharing on nearby Russian forces, technology for cyber defense, and systems for secure command and control. Antiarmor and antiaircraft weapons would help these countries deter and defend against invasion. America should provide them to deter Russian forces poised along the Ukrainian border. Media reports says Ukraine's interim government asked for US weaponry, but the request was deferred. A CNN poll in early March found that just over three-quarters of US respondents oppose providing military support to Ukraine, no doubtreinforcing Washington's hesitancy. As the consequences of Russian aggression sink in, and if it widens, sentiments may change.

To build further confidence in NATO’s collective-security commitments to members in its eastern area, Washington should return to Europe a third brigade combat team. If requested by Poland, it ought to be based there. Anxiety there is high – andwith reason. In suspending Russia’s observance of the Conventional Forces in Europe Agreement in 2007, President Vladimir Putin expressed a possible need to mass conventional forces on the border of new NATO members, particularly Poland. An augmented NATO position in Polandnow would bolster deterrence and incur fewer risks than if in the future Putin follows through on his warning.

Non-Black Sea NATO states should increase their warship rotational presence in the Sea, consistent with the Montreaux Convention limitation on each of them of three ships and 21-day stays. While respecting Turkish sensitivities, NATO might review the relevance of the Convention’s limits. Warship deployments could support NATO contingency planning for any escalation of Russian aggression, such as the use of its Black Sea Fleet to intimidate shipping that uses Odessa and other ports of littoral states.

At its summit in September, NATO ought to deepen its ties with Georgia, which seeks to join NATO, and Ukraine, which might now want a closer link or membership. Georgia has met key criteria for defense reform, civilian control, and free and fair elections. America's multi-year train-and-equip program has sharpened Georgian military skills and confidence. NATO members have conducted many years of military training and exercises with Ukraine.

In response to Russian aircraft flights near Turkey, its air force has scrambled fighters to patrol the Black Sea coast. NATO could support sustained joint air patrols over the Sea, led by Turkey and together with Romania, Bulgaria and Georgia.

The West should employ military power to deter Russian aggression against neighbors, but there is no reason to believe that Moscow plans nuclear attacks. While Western responses need not encompass nuclear-related countermeasures, no more withdrawals of US nuclear weapons from Europe should take place.

Prudent but potent Western defense steps will increase uncertainty in Kremlin thinking and unease about consequences of aggression in Crimea or beyond. They will also provide more space for diplomacy and political resolution. Two decades after the collapse of the USSR, a strong defense remains essential to achieving a Europe that is whole, free and at peace.

Job Henning is Nonresident Senior Fellow at the Center for the Study of the Presidency and Congress and Truman Project Fellow. William Courtney was US ambassador to Kazakhstan and Georgia, and special assistant to the President for Russia, Ukraine, and Eurasia.