The Taliban has stunned even some seasoned military and national security officials in the U.S. government with the speed of its conquests over the past week, sources with direct knowledge of the developments tell Axios.

Why it matters: President Biden isn't budging — resolved to get out by Aug. 31, no matter what — people briefed on his thinking say. He may not see much of a pause between his total withdrawal from Afghanistan and the country's total collapse into a bloody civil war.

Driving the news: "I do not regret my decision," Biden told reporters Tuesday. "We spent over a trillion dollars, over 20 years. We trained and equipped with modern equipment over 300,000 Afghan forces ... they've got to fight for themselves."

• But the Taliban has toppled seven provincial capitals in five days and now controls an estimated 65% of the country.
• The Taliban is striking with impressive speed and coordination, a senior Biden official grudgingly acknowledged. Afghans and international security are scrambling to manage mayhem.
• White House officials have privately reassured themselves by noting that polls show an overwhelmingly majority of Americans support withdrawing troops from Afghanistan.

Behind the scenes: Senior U.S. officials privately express little confidence in the Afghan security forces, citing military incompetence, disorganization and poor communications skills rendering them unable to adequately coordinate U.S. air support to protect territory against the Taliban.

• It's unclear what advice Chairman of the Joint Chiefs of Staff Gen. Mark Milley will give to President Biden on whether the U.S. should continue airstrikes against the Taliban after Aug. 31. The Pentagon declined to comment.

A former senior U.S. intelligence officer with extensive experience in the region said even if Biden were to later want to change course and authorize a significant post-withdrawal air campaign, doing so from outside Afghanistan's borders would be expensive and logistically difficult.

• The source, who remains in touch with his former colleagues, said they were despondent and had accepted that Biden is "dug in" and that resistance is "futile."

Pentagon spokesman John Kirby, at a press conference on Monday, said of the Afghan security forces that "it's their country to defend now; this is their struggle."

What they're saying: Sen. Ben Cardin (D-MD), who serves on the Senate Foreign Relations Committee, told Axios he discussed the situation in Afghanistan on Monday with Secretary of State Tony Blinken, and there's "no chance" Biden changes his troop withdrawal strategy.

• The committee's chairman, Sen. Bob Menendez (D-N.J.), told Axios he respects Biden but disagrees vehemently with the plan. "He doesn't want to have an endless war; I get that," Menendez said. "But I always thought that a contingency would have stemmed the tide... the president has to consider whether what's happening is what he envisioned."
• Still, many Democrats support the withdrawal. Sen. Chris Murphy (D-Conn.) told Axios: "We need to be able as a nation to pivot to other contests that matter more than Afghanistan… this is proof positive that Afghanistan is beyond saving."

The intrigue: A Biden State Department spokesperson told Axios that Zalmay Khalilzad, the U.S. envoy, is in Doha, where the Taliban has its political office, to "press the Taliban to stop their military offensive and to negotiate a political settlement."

• "If the Taliban continue down this path," the spokesperson added, "they will be an international pariah without support from the international community or even the people they say they want to govern."
• Asked about that, the Taliban spokesman Zabihullah Mujahid told Axios: "We have never yielded to any foreign pressure tactics before and we do not plan to capitulate any time soon either."

Between the lines: People who've worked at the highest levels on this issue say it's long past time to admit the peace process has failed.

• Lisa Curtis, a former senior NSC official who sat alongside Khalilzad during the Trump administration's negotiations with the Taliban, called on the Biden administration to "end its feckless calls for the Taliban to engage in peace talks when it's crystal clear they have no interest in a peaceful settlement."
• "Instead, the administration should lead an effort at the UN to slap sanctions on key Taliban leaders for the atrocities they are committing against civilians, such as the assassination of the government media chief last Friday.
• "While such sanctions may have little material impact, they would help deny the Taliban international legitimacy," she added. "On the other hand, standing by idly and doing nothing but pleading with the Taliban to enter negotiations only serves to legitimize them and their brutal behavior."

Alayna Treene and Margaret Talev contributed to this report.