19 January 1973
On 19 January 1973, South African Prime Minister J.B. Vorster confirms that there was no prior consultation with his government over Rhodesia's closure of its border with Zambia, but that his government will assist in fighting against terrorism. The closure of the border came after an accusation by Rhodesian (now known as Zimbabwe) Prime Minister Ian Smith that Zambia was hiding terrorists. On the same day,  the Zambian government alleged that Rhodesia might use an incident in which Zambian troops and police opened fire on a threatening South African police motorboat on the Zambezi River as a pretext for launching a military attack. On 24 January Zambia requested an urgent session of the United Nations’ Security Council, where it accused Rhodesia and South Africa of violating Zambia’s independence and integrity.The border between the two countries was closed on the 9 January 1973, preventing all movement except copper exports.
References

O’Malley P. ‘1973’, from Nelson Mandela Foundation, [online] available at www.nelsonmandela.org.za [Accessed: 03 December 2011]|

Bretcher M. & Wilkenfeld J. (1997) ‘A Study of Crisis’, [The University of Michigan], pg 250