As of mid-2018, it's been almost 17 years since the September 11, 2001 terrorist attacks that destroyed the Twin Towers in New York and damaged the Pentagon. Nearly 3,000 people died in the attack. Since that time, fortunately, there hasn't been another attack on the U.S. homeland that's anywhere close to the size and scale of 9/11.

However, we cannot rest easy.

As The Atlantic reported in September 2016:

Are we safer? Yes, we’re safer from the kind of orchestrated attack that shocked us on that September morning. It’s harder for terrorists to get into the country, and harder for them to pull off something spectacular if they do. But we have not plugged some of the most threatening security gaps.

A special report compiled by the Heritage Foundation examined 60 terrorist plots that have unfolded since 9/11.

How long can our luck – and the good work of law enforcement – hold out?

Can we prevent a US terrorist attack equal to (or worse than) 9/11 in terms of lives lost, at least through the year 2030?

For these purposes, a terrorist attack will point to something purposeful but not directly implemented by a nation-state's government and military.