U.S. Defense Secretary James Mattis has arrived in Kabul, Afghanistan, amid a national day of mourning following the deadliest Taliban attack on an Afghan military base in the last 16 years. On Friday, at least 140 Afghan soldiers were killed after Taliban militants, disguised as soldiers, drove onto a military base and opened fire on soldiers leaving Friday prayers. Some officials say as many as 200 soldiers died. The Afghan defense minister and army chief of staff have resigned in the wake of the attack.