Sen. Rand Paul (R-Ky.) said that if he were president, he would not let Russian President Vladimir Putin get away with his invasion into Ukraine.

Paul on Monday described the movement of Russian troops into the Crimean Peninsula as a gross violation of Ukrainian sovereignty, and argued it is the United States’ responsibility to take the lead in condemning it.

He also criticized President Obama’s policies toward Russia, arguing they have emboldened Russian President Vladimir Putin.

“The real problem is that Russia’s president is not currently fearful or threatened in any way by America’s president, despite his country’s blatant aggression,” Paul wrote in a Time magazine op-ed. “But let me be clear: If I were president, I wouldn’t let Vladimir Putin get away with it.”