When armies break / collapse, it can happen quickly. Here's a description of how rapidly the South Vietnamese army collapsed :

The 1975 spring offensive (Vietnamese: chiến dịch mùa Xuân 1975), officially known as the general offensive and uprising of spring 1975 (Vietnamese: Tổng tiến công và nổi dậy mùa Xuân 1975), was the final North Vietnamese campaign in the Vietnam War that led to the capitulation of Republic of Vietnam. After the initial success capturing Phước Long Province, the North Vietnamese leadership increased the scope of the People's Army of Vietnam's (PAVN) offensive and captured and held the key Central Highlands city of Buôn Ma Thuột between 10 and 18 March. These operations were intended to be preparatory to launching a general offensive in 1976.

Following the attack on Buôn Ma Thuôt, the Republic of Vietnam realized they were no longer able to defend the entire country and ordered a strategic withdrawal from the Central Highlands. The retreat from the Central Highlands, however, was a debacle as civilian refugees fled under fire with soldiers, mostly along a single highway reaching from the highlands to the coast. This situation was exacerbated by confusing orders, lack of command and control, and a well-led and aggressive enemy, which led to the utter rout and destruction of the bulk of South Vietnamese forces in the Central Highlands. A similar collapse occurred in the northern provinces.

Surprised by the rapidity of the ARVN collapse, North Vietnam transferred the bulk of its northern forces more than 350 miles (560 km) to the south in order to capture the South Vietnamese capital of Saigon in time to celebrate their late President Ho Chi Minh's birthday and end the war. South Vietnamese forces regrouped around the capital and defended the key transportation hubs at Phan Rang and Xuân Lộc, but a loss of political and military will to continue the fight became ever more manifest. Under political pressure, South Vietnamese President Nguyễn Văn Thiệu resigned on 21 April, in hopes that a new leader that was more amenable to the North Vietnamese could reopen negotiations with them. It was, however, too late. Southwest of Saigon IV Corps, meanwhile, remained relatively stable with its forces aggressively preventing VC units from taking over any provincial capitals. With PAVN spearheads already entering Saigon, the South Vietnamese government, then under the leadership of Dương Văn Minh, capitulated on 30 April 1975.

https://en.wikipedia.org/wiki/1975_spring_offensive

Experienced military commenters, like Scott Ritter, have been saying that they think such a collapse for the NATO Ukraine proxy army is not too far off. And as the South Vietnamese and countless others have shown before them, a collapsing army in chaotic retreat is easy pickings for the vanquisher.