The U.S. needs to take a harder line against Russia, said Sen. Tom Cotton, whose name is being batted around as a possible defense secretary.

Speaking at the Defense One Summit in Washington Thursday, Cotton blamed President Barack Obama for allowing Russia to annex Crimea, put troops into Ukraine, and generally behave more aggressively.

“I believe Donald Trump will be viewed, not just by Putin but by other world leaders, as taking a tougher line” against Russia and in defense of American security, said Cotton, a member of the Senate Armed Services Committee.

(But the president-elect has spoken admiringly of Russian Vladimir Putin and "has spoken of his desire to improve tattered U.S.-Russia ties," as Reuters put it. Retired Lt. Gen. Mike Flynn, a close campaign advisor who reportedly will be Trump's national security adviser, "appeared with Putin last year at an anniversary party for the Kremlin-controlled RT television network in Moscow," as reported by Politico.)

Cotton said Russia “has to have a new set of boundaries. They have to recognize that we are going to stand by our alliance structures.”

His interviewer, New York Times reporter David Sanger, noted Trump’s statements that he might not defend some allies unless they spent more on defense.

Cotton responded, “The best way to deter [Russian aggression] is to be ironclad in our support of our allies.” And he added, “It would be a good thing if our European allies were spending more than 2 percent because many have let their military capabilities wither over the past two decades.”

The senator also underlined his support for boosting spending to modernize U.S. nuclear weapons, which have “been a critical component of world peace” over the past 70 years. China and North Korea are expanding their nuclear arsenals, while Iran, “in my opinion,” is still on the path to nuclear weapons, he said.

“We need all three legs of the nuclear triad,” he said, and lauded ICBMs in particular as “a huge advantage” because “airplanes can be shot down and subs can be sunk,” and because it would require the enemy to have many and precise weapons to destroy silo-based missiles. He also suggested that perhaps the B-52 should be retired and replaced entirely by the planned B-21 bomber.

Cotton also said the U.S. should “rapidly expand our ballistic missile defenses on the West Coast and in the Pacific.” Longer term, he said, “we should have the goal of stopping an attack from a near-peer.”

Sanger said that BMD systems are improving but still have a relatively low hit rate.

Cotton responded that the U.S. needs more BMD weapons and gear: “I am confident that we have the technology to stop a North Korean attack. I am not sure we have the capacity.”

Finally, Sanger asked: How well has the Obama administration invested in and used cyber weapons?

“We could do more, I’ll just say that,” Cotton said.