Secretary of State John Kerry John Forbes KerryDivided country, divided church TV ads favored Biden 2-1 in past month Feehery: How Trump wins MORE warned in his exit memo on Thursday that the fight to defeat the Islamic State in Iraq and Syria (ISIS) is a "generational challenge" that won't be won "overnight."

"The fight against ISIL [Islamic State in Iraq and the Levant] and violent extremism more broadly is a generational challenge that cannot be won overnight," Kerry said. "It will require efforts on the battlefield and online, today and in years to come. But I am absolutely confident that, working with our coalition partners, we will win this fight and ISIL will be defeated."

Kerry in the memo detailed some of the challenges facing President-elect Donald Trump Donald John TrumpTrump says he doesn't think he could've done more to stop virus spread Conservative activist Lauren Witzke wins GOP Senate primary in Delaware Trump defends claim coronavirus will disappear, citing 'herd mentality' MORE, including the North Korean nuclear program, saying it is one of the country's "gravest" threats.

ADVERTISEMENT

Kerry also praised the accomplishments of the Obama administration, highlighting climate change. He stressed it should be one of top priorities of the Trump administration.

"Climate change is not just a threat to the future of our planet, but a growing and immediate threat to our national security and the lives and livelihoods of people across the globe," he argued.

Kerry also argued against cutting funding of the State Department.

"After serving in public life for nearly four decades, I am aware that there are few more reliable — or damaging — applause lines than promising to slash the budgets of the State Department and [United States Agency for International Development] and 'spend the money at home.' Good applause lines don’t always make good policy," Kerry said, adding that modest investments in diplomacy often result in significant returns in the future.

"The richest economy in the world cannot be content on putting only one penny on the dollar into this effort," he added.

Kerry concluded by wishing luck to Trump's pick to succeed him, Rex Tillerson, while adding that it has been the "honor of my life" to serve as the secretary of state.

"I send President Obama’s successor, and mine, all the best wishes as they embark on what will be a truly extraordinary journey representing the greatest country on earth," Kerry concluded.