Manu enumerates five Dharmas of Kshatriyas. Among these five Dharmas he did not mention the important Dharma of Kshatriya that is Fighting. Why he left that? He also left one important Dharma of Brahmins that is Tapasya. Why he left this Tapasya as Dharma of Brahmins. Manu feels that Tapasya and Fighting are the Natural characteristic of Brahmin and Kshatriya respectively. However, Manu talks about this nature in the later Chapters. When he talks about Aashauca, he says that there is no Aashauca for a Kshatriya if his relatives die in the battle field strictly adhering to his own Kshatriya Dharma i.e Fighting. Here is the verse - उद्यतैराहवे शस्त्रैः क्षत्रधर्महतस्य च । (मनु.५-९८). In the same way, he says a Brahmin is relieved from Aashauca by touching Waters and Kshatriya is relieved from Aashauca by touching the weapons and chariot. विप्रः शुद्ध्यत्यपः स्पृष्ट्वा क्षत्रियो वाहनायुधम् । (मनु.५-९९). By this Manu emphasizes that the utmost Dharma of Kshatriya is facing the battle field. Again, in praising Kshatriyas, Manu says that Kshatriya is made of the combination of Indra, Anila, Yama, Arka, Agni, Varuna, Candra and Kubera. Here, commentators write that Anila stands for Kshatra Dharma i.e. fighting. See the verse - इन्द्रानिलयमार्काणामग्नेश्च वरुणस्य च । चन्द्रवित्तेशयोश्चैव मात्रा निर्दृत्य शाश्वतीः॥(मनु.७-४). Again Manu describes that Kkshatriya could not be faced by any one, even seeing eye to eye. Thus, this verse - तपत्यादित्यवच्चैष चक्षूंषि मनांसि च । न चैनं भुवि शक्नोति कश्चिदप्यभिवीक्षितुम् ॥ (मनु.७-५) projects the Kshaatra Dharma. Further, Manu says that no one should irritate Kshatriyas and get his hatred. If anyone does so, his entire race would be destroyed by the Agni of Kshatra Dharma. Here is the verse - कुलं दहति राजाग्निः सपशुद्रव्यसञ्चयम् ॥ मनु-७-९). Moreover, Manu says that a Kshatriya shows his Vishwaruupa for the sake of protecting Dharma. It is his nature to protect Dharma by his Kshaatra Dharma. It would be clear from this verse - कुरुते धर्मसिद्ध्यर्थं विश्वरूपं पुनः पुनः ॥ (म्नु.७-१०).
Yama (Mrityu) is residing in the angry of a Kshatriya, which is all powerful. Kshatriya is always ready for fighting if there is anything goes against Dharma. Thus says this verse - मृत्युश्च वसति क्रोधे सर्वतेजोमयो हि सः ॥ (मनु.७-११). He even prescribes how a Kshatriya should make his Fort powerful. Manu says that a King should keep such a fighter on his Fort who can fight (kill) hundred fighters of enemy-side. And a King should have such 100 fighters who could fight with 10,000 enemy-soldiers. See this verse - एकं शतं योधयति प्राकारस्थो धनुर्धरः । शतं दश सहस्राणि तस्माद्दुर्गं विधीयते ॥ (मनु-७-७४). Thus a king should have various wealth of weapons, wealth, and chariots. तस्मादायुधसम्पन्नं धनधान्येन वाहनम् । (७-७५).
A King who is challenged by another King who may be superior, equal or inferior, should accept the challenge and do not come back defeated keeping Kshaatra Dharma in mind. It would be clear from this verse - समोत्तमाधमैः राजा त्वाहूतः पालयन्प्रजाः । न निवर्तेत संग्रामात् क्षात्रं धर्ममनुस्मरन् ॥ (७-८७). Manu emphatically says that not returning back accepting defeat, protecting the public, serving the Brahmins are the qualities for the welfare of the King. Thus this verse explains - संग्रामेष्वनिवर्तित्वं प्रजानां चैव पालनम् । शुश्रूषा ब्राह्मणानां चैव राज्ञां श्रेयस्करं परम् ॥ (७-८८). If two kings are fighting each other with the wish to win over each other, undefeatedly fighting attain Svarga. That is the nature of the Kings. See the verse -आहवेषु मिथोऽन्योन्यं जिघांसन्तो महीक्षितः । युध्यमानाः परं शक्त्या स्वर्गण् यान्त्यपराङ्मुखाः ॥ (७-८९).
Thus, fighting or facing the enemy in the battle field is the nature of Kshatriyas. This is evident in many places of Manusmriti.