Because it belongs to the same socialist countries, and Vietnam and China are still neighbors, China has provided assistance to Vietnam in all aspects, and when the United States wanted to further expand its regional influence, Vietnam was unlucky to be selected by the United States. So the United States began to help the South Vietnamese government, in Vietnam with the North Vietnamese government to form a confrontation, and the United States mainly hopes to plant a nail in Asia through this war, after the South Vietnamese occupation of Vietnam, let Vietnam establish certain ties with itself, and help the United States to further expand in Asia. However, the North Vietnamese government was not a soft bone at all, and China, as a neighbor of Vietnam at that time, began to sympathize with Vietnam after noticing Vietnam's situation, so China began to provide strategic support to Vietnam, not only supporting North Vietnam in its tenacious struggle with the United States, but also in order to make Vietnam have the strength to resist the United States, it sent military advisers to Vietnam, and finally assisted North Vietnam in unifying Vietnam. However, with the change of the world pattern, Vietnam began to make enemies with China, and then gradually became close to the Soviet Union, and after establishing a peaceful coexistence clause with the Soviet Union, it received a lot of assistance from the Soviet Union, coupled with Vietnam's previous defeat of the United States, which made Vietnam begin to delusionally dream of China's vast land, so after seeing the Soviet Union suppressing the north of the mainland, it began to continuously encroach on China's borders, causing great trouble to the border people on the mainland. However, the reason why Vietnam dared to blatantly invade China was also because the mainland's relations with the Soviet Union at that time fell to a freezing point, and Vietnam was in the same camp as the Soviet Union at that time, plus it and the Soviet Union just happened to be in the north to contain China.
However, because China knows the pain of war, it has always adhered to the principle of peaceful development, has always been secretive about war, and has not been willing to directly solve it by force for Vietnam, and has always held the idea of first saluting and then fighting, so China hoped to negotiate peace with Vietnam at that time and strive to solve this problem at the negotiating table, but Vietnam simply ignored the mainland's request, and even felt that China was afraid of things, so it began to frantically encroach on China's borders and create a lot of bloodshed. China itself loves peace, but is not afraid of war, so it launched a self-defense counterattack against Vietnam, attacking Vietnam, China took Hanoi directly in less than a month, after which China did not go any further, but chose to withdraw.
Of course, China did not say that it did not dare to go to war directly with Vietnam, mainly because China's situation was also more embarrassing at that time, when Sino-Soviet relations deteriorated, making China somewhat isolated and helpless in the world, just at this critical juncture, Vietnam began to engage in armed conflict with the mainland, it can be seen that the Soviet Union is actually behind the scenes, so if it directly chooses to fight with Vietnam, perhaps the Soviet Union in the north will be restless, and will directly order troops to invade China from the north. At that time, in order to fight Vietnam, the mainland also submitted a war application to the United Nations, and in the process, the Soviet Union continued to obstruct, and few countries in the world at that time expressed support for China. However, even in such a situation, a war with Vietnam is inevitable, and a war must break out between China and Vietnam, otherwise Vietnam will not give up, so China is determined to start a war. However, as far as the situation at that time was concerned, there was a strange situation, although the mainland's invasion of Vietnam has been highly opposed, but these countries only do verbal work, why is this situation happening?
At that time, the Soviet Union itself had thought of assisting Vietnam in attacking China, although the mainland and the Soviet Union had bad relations, but the relationship between the mainland and the United States was quite good, and China and the United States had always had exchanges, so when President Carter learned about the situation, he made a phone call to the Soviet Union and warned the Soviet Union that it was better to keep to itself, otherwise the world pattern might change. It was also because of this phone call that the Soviet Union eventually only sent a military advisory group to Vietnam, and as for the other small countries, they hoped that it would not affect themselves, so they would not directly send troops to Vietnam.
At that time, there were only two countries in the world that openly supported the mainland - Vietnam and Cambodia, and although these two countries did not provide substantial assistance to the mainland at that time, their spiritual support for the mainland was still worth mentioning.