open door

. (a) Open or free admission to all; hospitable welcome; free opportunity. [ Webster 1913 Suppl. ]

She of the open soul and open door,
With room about her hearth for all mankind. Lowell. [ Webster 1913 Suppl. ]

(b) In modern diplomacy, opportunity for political and commercial intercourse open to all upon equal terms, esp. with reference to a nation whose policy is wholly or partially fixed by nations foreign to itself, or to territory newly acquired by a conquering nation. In this sense, often used adjectively, as, open-door system, open-door policy, etc. [ Webster 1913 Suppl. ]

The steps taken by Britain to maintain the open door have so far proved to be perfectly futile. A. R. Colquhoun. [ Webster 1913 Suppl. ]