Wang Changling is famous for writing frontier poems. Please write one and tell me your understanding.

Seven Poems of Joining the Army (IV) Wang Changling Qinghai Changyun Dark Snow Mountain, lonely city overlooking Yumenguan.

Yellow sand wears golden armor in hundreds of battles, but the loulan is not returned. Enjoy the long clouds on Qinghai Lake and the snow-capped mountains stretching thousands of miles in Hubei. Across the snow-capped mountains, it is an isolated city, standing in the desert of Hexi Corridor. Further west, it is Yumenguan, a military fortress far away from the lonely city. This long scroll of thousands of miles in a vast area was the living and fighting environment of the northwest border guards at that time. The strong enemies of the northwest frontier in Tang Dynasty were Tubo and Turkic. Our mission to Hexi is to cut off the traffic between Tubo and Turkic and protect the Hexi Corridor. "Qinghai" area is the place where Tubo and Tang Jun fought many times; Outside Yumenguan, it is the sphere of influence of Turks. Therefore, these two sentences not only describe the scene of the whole northwest border, but also point out the extremely important geographical situation that the "isolated city" rejects Tubo in the south and defends Turks in the west. The strong enemies in these two directions are the hearts of the soldiers guarding the "lonely city", and Qinghai Lake and Yumenguan appear on the screen perfectly. This is not so much what the soldiers saw, but rather a picture that emerged in their minds. These two sentences are permeated with rich and complex feelings while writing the scenery: the frontier soldiers' concern for the frontier situation, their pride and sense of responsibility for their tasks, and the loneliness and hardship of frontier life are all integrated in the tragic, open and misty scenery.

In a word, "yellow sand covered with golden armor is invincible", the long time of defending the border, the frequent wars, the hardships of fighting, the toughness of the enemy and the desolation of the border are all summed up in these seven words. "Hundreds of Battles" only talks about many wars, but "Yellow Sand" highlights the characteristics of the battlefield, making people want to see the scene of "the ancient battlefield with sunset and sand like clouds". From "winning every battle" to "wearing golden armor", we can imagine the hardships and fierceness of the battle, and it is easy to imagine the tragic scene of sacrifice on the battlefield. Even so, the soldiers' ambition to serve the country has not been worn away, but they have become more determined in the desert sandstorm: "Never break Loulan", which is the heroic oath of the soldiers who have fought many battles. While expressing the lofty sentiments and ambitions of the soldiers on the frontier, they did not shy away from the hardships of the war, which is a major feature of this poem and a common feature of frontier poems in the prosperous Tang Dynasty. The high unity of typical environment and characters' emotions is a prominent advantage of Wang Changling's quatrains, which is also clearly reflected in this paper.