What was the Bush administration's or Republican party's explanation on the justification of Iraq invasion to the public when they failed to find Iraq's WMDs?
Wikipedia has a pretty good summary of the intelligence failure by the CIA that led in part to the Iraq invasion
According to a report by veteran investigative journalist Bob Woodward in his book Plan of Attack, Tenet privately lent his personal authority to the intelligence reports about weapons of mass destruction (WMDs) in Iraq. At a meeting on December 12, 2002, he assured Bush that the evidence that Iraq had WMDs amounted to a "slam dunk case." After several months of refusing to confirm this statement, Tenet stated that it was taken out of context. He indicated that it was made pursuant to a discussion about how to convince the American people to support invading Iraq. The search following the 2003 invasion of Iraq by US, British and international forces yielded no significant WMDs.
In September 2002, the Senate Intelligence Committee met with Tenet in a closed-door session. Sen. Bob Graham requested a National Intelligence Estimate (NIE) on Iraq. Tenet responded by saying "We've never done a National Intelligence Estimate on Iraq" and resisted the request to provide one to Congress. Graham insisted "This is the most important decision that we as members of Congress and that the people of America are likely to make in the foreseeable future. We want to have the best understanding of what it is we're about to get involved with." Tenet refused to do a report on the military or occupation phase, but reluctantly agreed to do a NIE on the weapons of mass destruction.
Congress voted to support the Iraq war based on the NIE Tenet provided in October 2002. However, the bipartisan "Senate Intelligence Committee Report on Prewar Intelligence" released on July 7, 2004, concluded that the key findings in the 2002 NIE either overstated, or were not supported by, the actual intelligence. The Senate report also found the US Intelligence Community to suffer from a "broken corporate culture and poor management" that resulted in a NIE which was completely wrong in almost every respect.
Citing "personal reasons," Tenet submitted his resignation to President Bush on June 3, 2004. He officially left on July 11, exactly seven years after being appointed by Clinton. James Pavitt, his Deputy Director for Operations at the CIA, announced his resignation the following day, leading to speculation that the exit of both senior intelligence officials was related to the controversy over the September 11 attacks, alleged Iraqi WMDs, and the decision to go to war with Iraq.
There were no evidence of WMD in Iraq. They used to be but they were obsolete or had been used already by Saddam Hussein against the Kurds. UN inspectors were sent and reported exactly that. But it was down played. The administration chose war regardless telling people that they had some but they were hidden. The point of the war was that Bush believed Saddam was a threat to the US and the Middle East and should be removed. They used the WMD card to justify the war. They didn't have the authorisation of the security council(it was granted after I believe) The war was a direct strike in response to 9/11, to satisfy people that something was being done to find the perpetrators. As per comment please see below:
UN resolution 1441 demanded that Iraqis cooperate fully with the UN to let UN inspectors full access and report to the council on the claim. Progress was slow, too slow and the US decided to act early as they saw that as a breach of resolution 1441.
Operation Avarice was declassified a few years after the war showing that there were stockpiles of warheads and rockets containing degraded chemical agents similar to those used in the Iran-Iraq War. From 2005 through 2006 military intelligence discovered that the weapons—many in poor condition, some empty or containing nonlethal liquid, but others containing sarin with unexpectedly high purity—were in the possession of one Iraqi individual who remained anonymous.