US President Joe Biden has said Hamas and Russian President Vladimir Putin both want to "annihilate" neighbouring democracies.
The president addressed the nation from the Oval Office on Thursday after his whirlwind trip to the Middle East.
He said people were facing "an inflection point in history - one of those moments where the decisions we make today are going to determine the future for decades to come".
"If we walk away and let Putin erase Ukraine's independence, would-be aggressors around the world would be emboldened to try the same," he said.
"The risk of conflict and chaos could spread in other parts of the world - in the Indo-Pacific, in the Middle East - especially in the Middle East."
"Hamas and Putin represent different threats, but they share this in common. They both want to completely annihilate a neighbouring democracy," President Biden said.
The United Nations has said that more than 4,000 people have been killed in the latest Middle East conflict.
Estimates put that number at around 3,478 in Gaza and 1,403 in Israel.
Taoiseach Leo Varadkar said on Thursday he has 'huge sympathy' for the Palestinian people who have been 'horribly treated' for the past 75 years.
Mr Varadkar told Newstalk Breakfast he believes Irish people have a particular sympathy towards the Palestinian people.
"We all have to be very careful about our own sympathies and prejudices," he said.
"I think Israel's a great country, I've had a chance to visit on a number of occasions, but I've huge sympathy for the Palestinian people.
"They've been horribly treated now for 75 years and denied self-determination and denied their own state.
"I think Irish people, given our own history, have a particular sympathy towards Palestinians," he added.
President Biden said he is to send an urgent budget request to Congress to fund and support Israel and Ukraine.
He added that making sure Israel and Ukraine succeed is vital to America's national security.
'Hamas does not represent Palestinians'
President Biden said he saw people who were "strong, determined, resilient and also angry" during his visit to Israel.
"I also spoke with President Abbas of the Palestinian Authority, and reiterated the United States remains committed to the Palestinian people's right to dignity and to self-determination," he said.
"The actions of Hamas terrorists don't take that right away."
President Biden said he believes Israel was not responsible for a blast at al Ahli Hospital in Gaza City on Tuesday, as Hamas officials have claimed.
He said he was "heartbroken" by the tragic loss of Palestinian life, "including the explosion at the hospital in Gaza, which was not done by the Israelis."
"We can't ignore the humanity of innocent Palestinians who only want to live in peace and have opportunity."
President Biden added: "Hamas does not represent the Palestinian people - Hamas uses Palestinian civilians as human shields and innocent Palestinian families are suffering greatly because of that."
'I refuse to let that happen'
President Biden went on to make the case for US involvement to ordinary Americans, saying some are asking, "Why does it matter to America" that the US support the wars?
"History has taught us that when terrorists don't pay a price for their terror, when dictators don't pay a price for their aggression, they cause more chaos and death and more destruction," he said.
"They keep going, and the cost and the threats to America and to the world keep rising.
"So, if we don’t stop Putin's appetite for power and control in Ukraine, he won't limit himself just to Ukraine."
He went on to say America's adversaries are watching how both conflicts play out and could stir up trouble elsewhere in the world depending on the outcome.
"We cannot and will not let terrorists like Hamas and tyrants like Putin win. I refuse to let that happen," he added.
Support for Ukraine
Ahead of his address, Mr Biden spoke with Ukrainian President Volodymyr Zelenskyy to stress that the US remained committed to backing Kyiv.
"The leaders discussed Ukraine's ongoing fight to defend its freedom and independence from Russian aggression," the White House said.
"President Biden underscored the continued strong bipartisan support in the United States for Ukraine's defense of its sovereignty, territorial integrity and democratic future."