President Bush wrapped up a speech in Moscow Friday to leaders of aid organizations at Spaso House, the official residence of the U.S. ambassador to Russia, where he praised the groups for working to help citizens and not a corrupt elite.

The speech followed the signing ceremony earlier in the day for the Treaty of Moscow, perhaps the centerpiece of his weeklong trip. The treaty calls for each country to trim its arsenal of more than 6,000 warheads to between 1,700 and 2,200 over the next 10 years. It also lets each country decide whether to dismantle the extra warheads or put them in long-term storage or use them as spares.

"Russia's a friend and that's the new thinking. That's part of what's being codified today," Bush said during the signing ceremony at the ornate St. Andrew's Hall.

Bush said the agreement puts the Cold War in the rearview mirror, but he struggled to explain why they still need the 2,000 nuclear warheads the treaty will allow them to keep now that the two nations are friends.

"You know, friends really don't need weapons pointed at each other. We both understand that. But it's a realistic assessment of where we've been. And who knows what will happen 10 years from now? Who knows what future presidents will say and how they'll react? If you have a nuclear arsenal, you want to make sure they work. And so, one reason to keep weapons in storage apart from launchers is for quality control," Bush said.

Putin also gave his reason for keeping a smaller nuclear arms supply. "Out there, there are other states who possess nuclear arms," he said. "There are countries that want to acquire weapons of mass destruction."

Putin had wanted the dismantled warheads destroyed, but he lost that end of the bargain. He also failed to get language in the treaty that would have put restrictions on the missile defense system Bush intends to build. He did, however, get the treaty codified in writing, a deal he has insisted since the two first began discussing details of the treaty last November, when Putin visited Bush's ranch in Crawford, Texas.

The Treaty of Moscow is the biggest arms reduction pact in history in terms of the amount of weapons it sidelines, and is expected to be ratified by both nations' legislatures.

U.S. officials say it may be the last such treaty of its kind, but some analysts downplay its importance since Bush was prepared to cut the U.S. arsenal on his own and Russia's economy was going to force cuts since it can't afford to maintain its current 6,000 warhead stockpile.

Meanwhile, the war against terrorism is overshadowing this symbol of the end of the Cold War. The president's main concern now is that chemical or biological weapons could fall into the hands of terrorist groups.

"The greatest danger in this war is the prospect of terrorists acquiring weapons of mass destruction. Our nations must spare no effort at preventing all forms of proliferation, and we discussed Iran in this context today. We will work closely with each other on this very important issue," Bush said.

As the two leaders signed the agreement, it was clear those concerns are causing new strains on the U.S.-Russian relationship. Bush's stated goal of overthrowing Iraqi President Saddam Hussein has not received a warm reception in Moscow. Iraq is heavily in debt to Russia, and Putin would like to prevent another war between Baghdad and the West. Bush also expressed concerns about Iran, which he fears is being supplied with nuclear assistance by Russia.

"We spoke very frankly and honestly about the need to make sure that a non-transparent government, run by radical clerics, doesn't get their hands on weapons of mass destruction," Bush said.

Putin said most of the assistance to Iran is energy-related, not unlike the help the United States has given to North Korea to build a nuclear power plant, a deal the United States made in exchange for North Korea's agreement to give up its nuclear weapons pursuit.

Iran, Iraq, and North Korea make up the "axis of evil" that Bush is trying to isolate. U.S. officials are concerned that the Bushehr reactor in Iran is being used for other than civilian needs. Putin added that Iran gets most of its nuclear aid from the West.

The presidents also issued a series of statements, agreeing to improve economic ties, work more aggressively for peace in the Middle East, allow more people-to-people contact, and cooperate closely on energy and counterterrorism.

Earlier in the day, Bush, Putin and the first ladies took a tour of Cathedral Square, which is the main square of the Kremlin. It is located between the Great Kremlin Palace and the Palace of Facets, and is surrounded by four cathedrals.

Bush has asked for ample exposure to Russian culture during his three-day visit to the country and his host is determined to provide it, both in Moscow and in Putin's hometown of St. Petersburg, where the two will go tomorrow. Putin is repaying Bush for the time he spent at the president's ranch.

Fox News' Wendell Goler and the Associated Press contributed to this report.