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In page László Bárdossy:

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On 2 April, the British warned the Hungarian ambassador that if Hungary allowed the transit of the Germans through her territory to attack Yugoslavia, Britain would break off diplomatic relations. They also cautioned Hungary that if she joined the attack on Yugoslavia under any pretext, she must expect Britain and her allies to declare war, and if that occurred, could expect to be treated appropriately should the Allies be victorious. The British observed that Hungary had renounced its claims on Yugoslav territory when it signed the Treaty of Friendship, and that any attack on Yugoslavia would be a flagrant breach of the treaty. These warnings were passed on to the Hungarian government by telegram.[19]