This image made from video of an undated still image broadcast in a news bulletin by North Korea's KRT on Monday, May 15, 2017, shows leader Kim Jong Un at what was said to be a missile test site at an undisclosed location in North Korea. KRT via AP Video As North Korea draws ever closer to possessing a nuclear weapon that could hit the US mainland, President Donald Trump and his top military advisers must weigh whether or not they'd launch a preemptive strike on North Korea and risk potentially millions of lives in the process.

But even though a US military strike on North Korea would be "tragic on an unbelievable scale," according to Defense Secretary Jim Mattis, that doesn't mean it's off the table.

At a National Committee on US-China Relations event in New York City, Samuel J. Locklear, the former head of the US military's Pacific Command made it clear: "Just because it's tragic doesn't mean he won't do it."

"If the national interests are high enough, and I think this is the mistake that [North Korean leader] Kim Jong Un needs really to think about, if you start pressing on an issue that has to do with the survival of the United States against a nuclear attack, the tragic becomes conceivable to stop it," said Locklear. "It could be tragic."

Adm. Timothy J. Keating, another former commander of Pacific Command, echoed Locklear's statement.

"There are a wide range of options" that are "readily available to the president and the secretary of defense resident in the planning warrens at Pacific command," Keating said at the event.

The discussion between two former top military commanders shows what a difficult situation the US is in with regard to North Korea. Pyongyang may wield up to 15 or so nuclear weapons, and they repeatedly threaten to use them against US forces, South Koreans, and Japanese.

Though the US has in place the world's most advanced missile defenses, there are no guarantees when it comes to stopping ballistic missiles. Even a single nuclear warhead touching down near Seoul could kill millions of innocent South Koreans in an instant.

Ballistic rocket is seen launching during a drill by the Hwasong artillery units of the KPA Strategic Force Thomson Reuters

Additionally, South Korea's new, progressive government would likely not approve of a military strike.

But the US has its own citizens to worry about. Experts contacted by Business Insider have spoken with near unanimity saying North Korea wants a thermonuclear-tipped intercontinental ballistic missile to hold the US at risk.

What exactly the US military planners discuss behind closed doors rightly remains classified, but if they calculate that a relatively small tragedy today could avert a massive tragedy tomorrow, then the US may see war with North Korea at some point.