While Kyoryu is right to point you to "fiction first", there is also the matter of fun.

In Fate, don’t get too bogged down trying to maintain absolute consistency in the world or adhere to a draconian sense of realism. The game operates by the rules of drama and fiction; use that to your advantage. There should be very few moments in the game where the PCs are free of conflicts or problems to deal with, even if it’d be more “realistic” for them to get a long breather.

When you’re trying to decide what happens, and the answer that makes the most sense is also kind of boring, go with something that’s more exciting than sensible! You can always find a way later on to justify something that doesn’t make immediate sense.

And remember to involve your players in the decision.

However, all of that said, lets take a look at the rules;

The conflict rules don't allow for this situation explicitly.

4. On your turn, take an action.

5. On other people’s turns, defend against or respond to their actions as necessary.

Specifically looking at the defend action:

Use defend when you’re actively trying to stop someone from doing any of the other three actions

If you are attacking, you generally are less likely to be also actively trying to defend yourself at that specific moment. This might suggest that a passive defence might in general be more appropriate, though I'd still refer you back to Kyoryu's answer to look at what exactly is happening.

Depending on the situation, you could roll against Mediocre (+0), as per the following rule from Fate Core:

If ... you want to forgo your defence ... Because you’re not defending, the attacker’s rolling against Mediocre (+0) opposition, which means you’re probably going to take a bad hit.

Though that could be modified with passive opposition from things such as cover and equipment.

While Kyoryu is right to point you to "fiction first", there is also the matter of fun.

In Fate, don’t get too bogged down trying to maintain absolute consistency in the world or adhere to a draconian sense of realism. The game operates by the rules of drama and fiction; use that to your advantage. There should be very few moments in the game where the PCs are free of conflicts or problems to deal with, even if it’d be more “realistic” for them to get a long breather.

When you’re trying to decide what happens, and the answer that makes the most sense is also kind of boring, go with something that’s more exciting than sensible! You can always find a way later on to justify something that doesn’t make immediate sense.

And remember to involve your players in the decision.

However, all of that said, lets take a look at the rules;

The conflict rules don't allow for this situation explicitly.

4. On your turn, take an action.

5. On other people’s turns, defend against or respond to their actions as necessary.

Specifically looking at the defend action:

Use defend when you’re actively trying to stop someone from doing any of the other three actions—you’re parrying a sword strike, trying to stay on your feet, blocking a doorway, and the like. Usually this action is performed on someone else’s turn, reacting to their attempt to attack, overcome, or create an advantage. You may also roll to oppose some non-attack actions, or to defend against an attack on someone else, if you can explain why you can. Usually it’s fine if most people at the table agree that it’s reasonable, but you can also point to an relevant situation aspect to justify it. When you do, you become the target for any bad results.

If you are attacking, you generally are less likely to be also actively trying to defend yourself at that specific moment. This might suggest that a passive defence might in general be more appropriate, though I'd still refer you back to Kyoryu's answer to look at what exactly is happening.

Depending on the situation, you could roll against Mediocre (+0), as per the following rule from Fate Core:

If ... you want to forgo your defence ... Because you’re not defending, the attacker’s rolling against Mediocre (+0) opposition, which means you’re probably going to take a bad hit.

Though that could be modified with passive opposition from things such as cover and equipment.

While Kyoryu is right to point you to "fiction first", there is also the matter of fun.

In Fate, don’t get too bogged down trying to maintain absolute consistency in the world or adhere to a draconian sense of realism. The game operates by the rules of drama and fiction; use that to your advantage. There should be very few moments in the game where the PCs are free of conflicts or problems to deal with, even if it’d be more “realistic” for them to get a long breather.

When you’re trying to decide what happens, and the answer that makes the most sense is also kind of boring, go with something that’s more exciting than sensible! You can always find a way later on to justify something that doesn’t make immediate sense.

And remember to involve your players in the decision.

However, all of that said, lets take a look at the rules;

The conflict rules don't allow for this situation explicitly.

4. On your turn, take an action.

5. On other people’s turns, defend against or respond to their actions as necessary.

Specifically looking at the defend action:

Use defend when you’re actively trying to stop someone from doing any of the other three actions

If you are attacking, you generally are less likely to be also actively trying to defend yourself at that specific moment. This might suggest that a passive defence might in general be more appropriate, though I'd still refer you back to Kyoryu's answer to look at what exactly is happening.

Depending on the situation, you might roll against Mediocre (+0), as per the following rule from Fate Core:

If ... you want to forgo your defence ... Because you’re not defending, the attacker’s rolling against Mediocre (+0) opposition, which means you’re probably going to take a bad hit.

Though that could be modified with passive opposition from things such as cover and equipment.

While Kyoryu is right to point you to "fiction first", there is also the matter of fun.

In Fate, don’t get too bogged down trying to maintain absolute consistency in the world or adhere to a draconian sense of realism. The game operates by the rules of drama and fiction; use that to your advantage. There should be very few moments in the game where the PCs are free of conflicts or problems to deal with, even if it’d be more “realistic” for them to get a long breather.

When you’re trying to decide what happens, and the answer that makes the most sense is also kind of boring, go with something that’s more exciting than sensible! You can always find a way later on to justify something that doesn’t make immediate sense.

And remember to involve your players in the decision.

However, all of that said, lets take a look at the rules;

The conflict rules don't allow for this situation explicitly.

4. On your turn, take an action.

5. On other people’s turns, defend against or respond to their actions as necessary.

Specifically looking at the defend action:

Use defend when you’re actively trying to stop someone from doing any of the other three actions

If you are attacking, you generally are less likely to be also actively trying to defend yourself at that specific moment. This might suggest that a passive defence might in general be more appropriate, though I'd still refer you back to Kyoryu's answer to look at what exactly is happening.

Depending on the situation, you could roll against Mediocre (+0), as per the following rule from Fate Core:

If ... you want to forgo your defence ... Because you’re not defending, the attacker’s rolling against Mediocre (+0) opposition, which means you’re probably going to take a bad hit.

Though that could be modified with passive opposition from things such as cover and equipment.