A treaty committing the United States and Russia to keep their long-range nuclear arsenals at the lowest levels since early in the Cold War goes into full effect on Monday. When it was signed eight years ago, President Barack Obama expressed hope that it would be a small first step toward deeper reductions, and ultimately a world without nuclear weapons.

Now, that optimism has been reversed. A new nuclear policy issued by the Trump administration on Friday, which vows to counter a rush by the Russians to modernize their forces even while staying within the treaty limits, is touching off a new kind of nuclear arms race. This one is based less on numbers of weapons and more on novel tactics and technologies, meant to outwit and outmaneuver the other side. Continue reading.