World wide airline and naval travel easily transport dangerous people and material. The Nephites must have been surprised at how narrow their strip of wilderness could be at times, our protection is just as thin if we do not set proper guards (Hel 1:18) or be "up and doing" in defense of our liberty(Alma 60:24).
During the Cold War we could nominally count on the international order to restrain the actions of our enemy. But even this existence led to the Cuban Missile Crisis and Krushchev threatening to "swat America's ass" with the weapons he inserted there. Now we face regimes that explicitly reject that world order, support terrorism as an arm of foreign policy, and seek the most devastating weapons known to man.
The threat is just as real and apparent as the Lamanites marching on Zarahemla. Yet if we wait for the launch of nuclear missiles, or a terrorist attack using the same, we will be lamenting the desolation of Ammonihah instead. Arguing for a neo isolationist foreign policy based on The Book of Mormon ignores the strategic realities that both nations faced as a result of geography and technology. The nature of modern technology, the connection of rogue regimes with terrorist organizations, the precedent re enforced by 9/11, and the shrinking world of globalization demand that pursue an "offensive defensive" like the Nephites of old.