If your specimen is a:and has stem­/­leaf ar­range­ment:and the leaves have:and the bark is/has:and the twigs:and the fruit (if present):then you have:
tree╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood genus Cornus
sub-opposite small ▲▲ teeth dark, lots of light-colored lenticels ;
inner bark=­or­ange (scrape with knife)
often have a terminal thorn are small, black, taste dis­gust­ing after a few seconds—spit it out lookalike common buckthorn Rhamnus cathartica
no more than waist-high (<1 m) (<3 ft) ╟ alternate ▲▲ teeth no hair lookalike alderleaf buckthorn Rhamnus alnifolia
tree ▲▲ teeth, visible within arm's length dark, lots of light-colored lenticels ;
if mature, looks covered with burnt potato chips­/­crisps
are small, black, taste like 🍒︎ cherry even if sour lookalike 🍒︎ black cherry Prunus serotina
___ smooth or
very small ▲▲ teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn ;
this plant, please continue below . . .

Verify with Google images.

Also called Rhamnus frangula.  Also called alder buckthorn (although do not confuse with alderleaf buckthorn Rhamnus alnifolia which is also called that) and breaking buckthorn.

Native to 🇪🇺 Europe, northern Africa and western Asia.  Invasive > learn+quiz Invasive > Global Invasive > in USA+Canada Invasive > report it! Invasive > in Michigan inv. trading card

Learn more about glossy buckthorn Frangula alnus

Best Control Practices BCP Discover Life Encyclopedia of Life Google Google images Michigan Flora USDA PLANTS db Wikipedia