Who competes with Walmart in the USA?

Here we have: Tesco, Asda, Morrisons, Sainsbury's, Aldi, Lidl, Waitrose, M&S.

In the UK nearly all towns have at-least 2 supermarkets, and most towns have another 3 within a 20 minute cycle/drive.

The fundamental problem with Amazon is as with Standard Oil. They have no competition, and they have no separation between their logistics/warehouse business & their marketplace. Thus, they are using their delivery business and warehouse arm (which in itself would not be sustainable) to ward-off competition. Just as when Standard Oil bought the railway companies.