BRUSSELS — Russian submarines have dramatically stepped up activity around undersea data cables in the North Atlantic, part of a more aggressive naval posture that has driven NATO to revive a Cold War-era command, according to senior military officials. We are now seeing Russian underwater activity in the vicinity of undersea cables that I don’t believe we have ever seen,” said U.S. Navy Rear Adm. Andrew Lennon, the commander of NATO’s submarine forces. “Russia is clearly taking an interest in NATO and NATO nations’ undersea infrastructure.”