In the U.S.A., it's against the law and a criminal offense just to try or to attempt to obstruct justice.  Just the attempt is a criminal offense.

And according to one news article, "James Comey says Trump asked him to drop Michael Flynn investigation in statement".

If that's true, wouldn't that be enough to impeach Trump for just attempting to obstruct justice?

later Trump fired Comey when he was leading the investigation into the Trump US/Russia conection.

Couldn't that also be considered an attempt to obstruct justice and enough to impeach Trump?

And, like I said, it doesn't have to be actual obstruction. It just has be an attemp to obstruct justice. The attemp alone is a criminal offense.