This is one of the reasons, beyond the constitutional requirement, Congress should be the one to declare wars. They also have the power of the purse and can, at the same time, allocate more funding toward the war effort. If Congress is willing to declare war, they'll generally be willing to fund it (at initiation or later). Since Trump bypassed Congress here, he's now going back and asking for funding for a cause that Congress did not actually approve of.
This is one of the reasons, beyond the constitutional requirement, Congress should be the one to declare wars. They also have the power of the purse and can, at the same time, allocate more funding toward the war effort. If Congress is willing to declare war, they'll generally be willing to fund it (at initiation or later). Since Trump bypassed Congress here, he's now going back and asking for funding for a cause that Congress did not actually approve of.