September 28, 2015 11:51 AM EDT

President Obama called on world leaders Monday to come together to temper and avert global conflicts, noting the spirit of diplomacy on which the United Nations was founded 70 years ago this year.

“We can bridge our differences and choose cooperation over conflict,” Obama said, speaking before the United Nations General Assembly. “That is not weakness, that is strength.”

For 45 minutes, Obama openly criticized nations and leaders that reject diplomacy and democracy, while touting the strength that the United States has gained as a result of allowing citizens to think, worship, and live freely within its borders. But to ensure the freedom of all people across the globe, Obama said, the U.S. cannot act alone.

“No matter how powerful our military, how strong our economy… the United States cannot solve the world’s problems alone,” Obama said, noting specifically that the use of force in Iraq did not end the nation’s strife.

Obama touted the nuclear deal his administration brokered with Iran as an example of “the strength of the international system when it works the way it should,” and held up the ongoing shift on Cuban policy as an example of diplomacy at work.

He justified the U.S. sanctions on Russia not as an effort to return to “Cold War politics” but as an example of the U.S. refusing to stand by when a sovereignty of a nation is violated. “If that happens without consequence in Ukraine, it can happen in any nation gathered here today,” Obama said.

He was openly critical of Russia’s annexation of Crimea and the ongoing strife in the region, but argued that the results of the country have backfired, causing Ukrainians to align with Europe, educated Russians to flee and the contraction of their economy.

“Imagine instead if Russia had engaged in true diplomacy,” Obama said. “That would be better not just for Ukraine, but better for Russia.” Obama’s statements came hours before he was scheduled to sit down with Russian President Vladimir Putin, their first meeting in nearly a year.

The President also forcefully denounced Syria’s leader Bashar Assad, who Obama called a “tyrant,” as well as the notion that leaders should support Assad in their efforts to avoid further strife in the Middle East. Putin is expected to make that case to the U.N. later on Monday.

“The people of our United Nations are not as different as they are told,” Obama said. “They can be made to fear, they can be taught to hate, but they can also be taught to hope.”

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