Russia's rising threat to nearby neighbors isn't confined to Ukraine. That menace demands fresh American assurances to the NATO allies that once were forced into the Soviet Union's sphere of influence.

The North American Atlantic Treaty Organization marked its 65th birthday last month. The political and military alliance has kept its outer form, but its military strength is a shadow of what it was when the Cold War ended in 1990.

Even so, it has added members that were once under Soviet occupation, including Poland, the Czech Republic, Slovakia, Hungary, Romania, Bulgaria, Latvia, Lithuania and Estonia. Several of those nations border Ukraine. Poland's eastern border is with Belarus, already a Russian client state. The three Baltic nations (Latvia, Lithuania and Estonia) all border Russia.

Russian President Vladimir Putin's current intentions are unclear. He has publicly urged pro-Russia separatists in eastern Ukraine to delay Sunday's scheduled referendum that could advance their zeal for independence from Kiev. Yet on Thursday those separatists made it clear that they intend to hold the referendum despite Mr. Putin's pleas.

However, Mr. Putin's ambition to revive the prestige and strength of the old Soviet Union is well known - and was on alarming display in March as Russia invaded and annexed Crimea. Thus, Russia's NATO member neighbors have good reason for concern.

President Barack Obama has taken a number of tentative steps that he describes as efforts to change Mr. Putin's calculus about how far it is safe to go.

The U.S. moves have included sanctions on individuals close to the Russian president and the dispatch of four companies of American troops, about 600 men, to Latvia, Lithuania, Estonia and Poland.

And NATO's Supreme Commander, U.S. Air Force Gen. Philip Breedlove, recently said alliance leaders had instructed him to set up "reassurance measures" involving the movement of air, land and sea forces to NATO's forward lines. He said those measures would last until Dec. 31, but added that NATO is considering stationing permanent forces on its eastern borders.

Still, without firmer U.S. leadership for a stronger NATO, European nations are not likely to bolster their defenses in the manner needed to deter Russian expansion.

Of course, after extended missions in Iraq and Afghanistan, the U.S. public is also understandably wary of potential military confrontations.

That means it will take a clear presidential commitment to shake off American isolation and persuade Russia that further expansionism would be a losing proposition.

President Obama should speed the export of American oil and gas to Europe to reduce its dependence on Russian energy. He - and Congress - should also slow the planned reduction of the defense budget. For instance, the scheduled retirement of the Air Force's tank-killer A-10 fighter should be deferred.

U.S. support for our NATO allies should be underlined in as many ways as possible.

President Obama has the opportunity and the responsibility to prepare the nation - and NATO - to meet Russia's challenge.

And the best way to discourage President Putin's expansionist urge is to demonstrate that the American-European alliance remains solid - and united against Russian aggression.