In 2001, my husband, Bill, and I had been married just under a year when we watched on TV as the Twin Towers fell. Next month, we will celebrate our 18th anniversary. It's hard to believe that it's been 17 years since 9/11, or that we've been married even longer than that, and yet both are true.

The towers were always part of my Manhattan landscape. For years, wandering the confusing streets of Greenwich Village, I'd instinctively look for the towers to the south and the Empire State Building to the north as reliable landmarks, visible from anywhere, to orient me. Viewing New York from a distance, those distinctive towers, dramatically higher than anything else, would automatically draw my eye, helping me zero in on Manhattan. For a long time, the island where I was born just didn't look like itself without them.

These many years later, two huge reflecting pools fill the footprints where the buildings once were and the Freedom Tower stands nearby, once again the tallest structure in the Western hemisphere. The city and the nation have rebuilt themselves in every sense of the word. But on the 17th anniversary of that unforgettable day, what have we learned from those attacks?

The U.S. isn't invulnerable.

One lesson is that the United States isn't the impregnable fortress we once thought it was. Most Americans generally feel safe from attacks by foreign powers when we're on American soil, and with good reason. The World Trade Center was struck twice, the first time in 1993 with a truck bomb that failed to bring down the towers but did kill six people, and the second time on September 11, 2001. Before those events, no foreign power had successfully attacked the United States since December 7, 1941, when Pearl Harbor was bombed in Hawaii. No foreign power had succeeded in attacking the continental United States since the Revolutionary War. Americans overseas have been targets at times, for instance in 1979 when employees at the U.S. embassy in Iran were taken hostage, or in 2000 when suicide bombers attacked the Navy vessel the USS Cole, killing 17 American sailors. But before 9/11 and since, Americans on their home territory have been safe from foreign attacks.

Every life is important.

I'm hoping this is a lesson we've learned or are learning. USA Today noted, as the 9/11 anniversary approached, that the 17-year-olds who sign up to join the U.S. military over the next few months will be the first group to do so when our nation has been at war their entire lives. The Afghan war began less than a month after the 9/11 attacks, after the Taliban, then ruling the country, refused to hand over Osama bin Laden or shut down the al Qaeda bases where the 9/11 hijackers had trained. 

The war continues today, with 15,000 U.S. troops still on the ground. A 17-year war takes a terrible toll, and not only on the U.S. and Afghan troops who are fighting it. Today, as we commemorate the 2,977 civilians who died on 9/11, it's also worth remembering that more than 10 times that number of Afghan civilians have been killed by that war. Though exact numbers are hard to tally, researchers at Brown University estimated the toll was roughly 31,000 from the beginning of the war through mid-2016. Other groups estimate a civilian death toll higher than 100,000. Some 2,200 U.S. troops have also been killed. USA Today argues that there are signs over the past few months that a negotiated peace is possible and so we should hold out a bit longer, while also publishing an opposing view that we should pull out without delay. 

I'm not certain who's right. But if we don't pull out by 2019, the U.S. war in Afghanistan will surpass the Vietnam War to become America's longest. That's not a record we should be eager to break.

Hope is better than anger.

It's still easy, 17 years later, to be angry about the September 11 attacks. I would never fault anyone who lost a loved one that day for feeling angry forever. But for me, the lessons of 9/11 are about survival, about resilience, about people coming together to help one another during a crisis, and about the U.S. as a nation of the world rather than one that stands alone.

At the beautiful National September 11 Memorial, where President Donald Trump is attending a commemoration today, my very favorite thing is a Callery pear tree known as the "Survivor Tree." Planted at the original World Trade Center in the 1970s, it was badly burned and damaged when the towers fell, reduced mostly to an eight-foot stump, its branches and roots broken off.

It amazes me that in October 2001, in the middle of clearing out the toxic and still smoking rubble at what was then called Ground Zero, workers had the presence of mind to notice that the tree was still alive and to ask the city's horticultural experts to try and save it. The city's Department of Parks & Recreation moved what was left of the tree to a site in the Bronx and nursed it back to health. In 2010, it was returned to a place of honor at the 9/11 Memorial.

Since then, it's been visited by President Obama and foreign heads of state. Mourners hung rainbow colors of ribbons all over it after the Orlando nightclub shooting two years ago. Seedlings from the tree have been sent to communities all over the world to commemorate other tragedies, most recently to Manchester, England, after a terrorist bombing killed 22 people at an Ariana Grande concert. 

Being a fruit tree, the Survivor Tree flowers every year, bursting into a cloud of white blossoms. I can't think of a better way to remember, and to look ahead.