Retired Gen. Stanley McChrystal told CNN Thursday that President Trump will own his Defense secretary's decision to send 4,000 additional troops to retake parts of Afghanistan from the Islamic State in Iraq and Syria (ISIS).

"I believe Jim Mattis will make a great decision on Afghanistan," McChrystal said hours before the announcement. "But I don't think that takes away President Trump's responsibility for that decision."

McChrystal, who served under former President Barack Obama Barack Hussein ObamaObama warns of a 'decade of unfair, partisan gerrymandering' in call to look at down-ballot races Quinnipiac polls show Trump leading Biden in Texas, deadlocked race in Ohio Poll: Trump opens up 6-point lead over Biden in Iowa MORE, told CNN's Jake Tapper that Trump's position as commander in chief means he will be held responsible for the outcome of Mattis's decision.

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"Ultimately, he is the commander in chief and the leader of the American people," McChrystal said. "So President Trump will ultimately own the outcome, so it's important we take a long view."

McChrystal broke with Mattis's decision to send 4,000 additional troops to supplement the about 8,400 troops already in the country, warning against the possibility of the U.S. footprint in the country becoming too large.

"I'm a believer in the Afghan people, so I support an increase in forces there," McChrystal said. "But the question we have to ask is ... does that [increase] produce the outcome we want?"

"I think there's going to have to be an adequate ground force, but I don't think it should be primarily American," McChrystal said. "That is a regional issue to which regional forces are most appropriate."

McChrystal warned that too many U.S. forces could easily backfire and benefit ISIS.

"I think we need to think in terms of what will be the reaction in the region if there's too big a footprint. So I really think again, what we need to think about is not just ISIS, but the condition of the region post-ISIS."