If the United States pulls completely out of Afghanistan, it could leave a vacuum that would be filled by "some combination of ISIS, al-Qaida, and the Taliban," former House Speaker Newt Gingrich said Monday, but he doesn't expect President Donald Trump will allow that to happen.

"We are in the long-term struggle with terrorism, with Islamist supremacists," Gingrich told Fox News' "Fox and Friends" program.

"We know that the last time we allowed Afghanistan to be an empty space, it was filled with the Taliban and by al-Qaida and the planning for 9/11 was done in Afghanistan."

However, Gingrich said that he expects Trump to announce in his national address Monday that there's a "prudent investment, probably 4,000-5,000 troops that allows us to train the Afghan army."

That, combined with a "new level of pressure" and leaving Pakistan to clean up the northwest frontier "could lead us over the next few years into a much better future," Gingrich said.

"I think you're going to see relatively fewer American casualties, you're going to see a real emphasis on training the Afghans, and I think in the long run that will be the right strategy," said Gingrich.

The former speaker said he also does not think anyone can accuse Trump of not thinking the matter of Afghanistan through, as he is working with a "superb national security team" in Secretary of State Rex Tillerson, Chief of Staff John Kelly, Defense Secretary James Mattis, and National Security Adviser Gen. H.R. McMaster.