It is reliant on the intensity of the attack as well as the power of the attacking nation.
some countries have been attacked and literally never fought back but surrendered if the attacking nation is more powerful in terms of military power.
in case the intensity of the attack can be absorbed, a country can also opt for diplomacy as war is the ultimate sanction in international relation.
in case the country feels it has the capacity to protect its sovereignty then fighting back is the only option.
another way to look at it would be one of the most difficult issues in foreign policy is deciding when the United States should exercise military force. Most people think that military force may be used if a vital national interest of the United States is threatened. The difficulty lies in getting people to agree on what constitutes a vital national interest.
Almost everyone would agree that an attack by a foreign country on the United States threatens a vital interest. Many also would think a vital interest threatened if a country attacked a nation that we had signed a security agreement with. Disagreements emerge when the threat involves the free flow of a precious commodity, such as oil. They also surface over situations that do not pose an immediate threat to U.S. security but could imperil it in the future, such as when a region becomes unstable and the instability may lead to wider conflicts. Another area of debate opens over human rights and humanitarian efforts. The United States is the most powerful democratic nation on Earth. Does that mean we always have a vital interest in promoting human rights and democracy? Or, should we stay out of the affairs of other nations unless they threaten other of our national interests?
Another issue arises over how the United States should exercise military force. Some argue that America should never act unilaterally, but should only act with others, allies or particularly with the United Nations. They believe America has a strong interest in upholding international law. Others agree that it is appropriate to act in coalitions, but they think demanding it in every circumstance would paralyze America’s role as a world leader.
