Upon encountering an enemy or those of authority

اللّهُـمَّ إِنا نَجْـعَلُكَ في نُحـورِهِـم، وَنَعـوذُ بِكَ مِنْ شُرورِهـمْ

O Allah, we ask You to restrain them by their necks, and we seek refuge in You from their evil.

Abu Dawud 2/89, and Al-Hakim graded it authentic and Ath-Thahabi agreed 2/142.

اللّهُـمَّ أَنْتَ عَضُـدي، وَأَنْتَ نَصـيري، بِكَ أَجـولُ وَبِكَ أَصـولُ وَبِكَ أُقـاتِل

O Allah, You are my strength and You are my support. For Your sake, I go forth and for Your sake, I advance and for Your sake, I fight.

Abu Dawud 3/42, At-Tirmidhi 5/572. See also Al-Albani, Sahih At-Tirmidhi 3/183.

حَسْبُـنا اللهُ وَنِعْـمَ الوَكـيل

Allah is sufficient for us and the best of those on whom to depend.

Al-Bukhari, 5/172.