boneh3ad is technically correct - perhaps I oversimplified and should have qualified my post. But the original question is essentially clarifying the impact of flow separation on drag and that impact is essentially on pressure drag.
The cancellation of front and back forces is what gave rise to D'Alembert's paradox - that there is no drag in inviscid flow while real flows do experience drag. Skin friction, induced and wave drag all have their place. but in practice, skin friction drag is usually a very small component of total drag, induced drag occurs only when the object generates tip vortices and wave drag occurs only at transonic and above speeds - I omitted mention of these other kinds of drag to focus on pressure drag.
"Weakening" is perhaps a poor choice of word to describe what is happening to the boundary layer. I meant the reduction in flow velocity that eventually causes flow reversal in the boundary layer, resulting in flow separation.
Boundary layers separate when faced with an adverse pressure gradient. If there is none e.g. a semi-infinite flat plate, then there is nothing to cause separation. But in practice, boundary layers over bodies in a flow do separate (assuming a viscous continuum fluid). (At subsonic speeds,) a fluid flow encountering a body "sees" an increasing cross-sectional area as it proceeds past the body until a maximum area is reached at the thickest part of the body. In this front portion of the body, streamlines converge as they proceed, flow velocity accelerates and pressure decreases. Past the maximum point, the flow sees a decreasing cross-sectional area until it reaches zero area at the point that the fluid exits the body. Streamlines diverge, flow velocity decelerates and pressure "recovers" (i.e. increases). Pressure recovery creates an adverse pressure gradient i.e. the fluid experiences increasing pressure as it proceeds. This adverse pressure gradient is inevitable for bodies because the body has to end at some point. The velocity in a boundary layer is already slow, especially near the surface, and an adverse pressure gradient causes it to slow down further (weaken?) to a point when the velocity reverses direction. Fluid is now flowing into the boundary layer between the surface and the boundary layer - and this is what is known as flow separation.
As the original question was referring to vehicles and wings, there will always be flow separation at some point "behind". The fluid that fills the space left by the separated flow has less pressure than a flow that stayed attached - and the missing pressure is what causes pressure drag.
So to restate what was meant about flow separation and the boundary layer: "... flow separation always occurs due to (the inevitable) formation of a boundary layer that [STRIKE]weakens [/STRIKE] will inevitably encounter an adverse pressure gradient as the flow goes over the back"
By design, wings have a gradual taper that minimizes the adverse pressure gradient, but "minimal" is still enough to cause flow separation except that it is delayed to occur near the trailing edge (at low angles of attack), allowing most of the flow around the wing to look like an inviscid flow. As in inviscid flow, the pressure over the rear portion of the wing cancels the frontal pressure up to where the flow separates and hence the pressure drag is very low if flow separation occurs at the thin end of the wing. Skin friction drag is unaffected and hence becomes significant when compared to the low pressure drag. At high (i.e. stalling) angles of attack, flow separation happens right up front near the thickest part of the wing - and drag increases sharply as pressure drag takes over.
Wings have induced drag too, but perhaps that is something for another discussion. Most other bodies (cars, balls, frying pans etc) would suffer from flow separation over a large part of the body if they flew through the air - but if a gently tapered shroud was added, the pressure drag would be reduced significantly.
