diff --git a/go/daemon/network/loader.go b/go/daemon/network/loader.go
index 4b1789c..d41403c 100644
--- a/go/daemon/network/loader.go
+++ b/go/daemon/network/loader.go
@@ -4,12 +4,14 @@ package network
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"isle/bootstrap"
 	"isle/daemon/children"
 	"isle/daemon/daecommon"
 	"isle/nebula"
 	"isle/toolkit"
+	"os"
 
 	"dev.mediocregopher.com/mediocre-go-lib.git/mlog"
 )
@@ -30,6 +32,26 @@ func networkRuntimeDir(
 	return networksRuntimeDir.MkChildDir(networkID, mayExist)
 }
 
+func cleanupDirs(
+	networkStateDir, networkRuntimeDir toolkit.Dir,
+) error {
+	var errs []error
+
+	if err := os.RemoveAll(networkStateDir.Path); err != nil {
+		errs = append(errs, fmt.Errorf(
+			"removing %q: %w", networkStateDir.Path, err,
+		))
+	}
+
+	if err := os.RemoveAll(networkRuntimeDir.Path); err != nil {
+		errs = append(errs, fmt.Errorf(
+			"removing %q: %w", networkRuntimeDir.Path, err,
+		))
+	}
+
+	return errors.Join(errs...)
+}
+
 func networkDirs(
 	networksStateDir, networksRuntimeDir toolkit.Dir,
 	networkID string,
@@ -268,6 +290,22 @@ func (l *loader) Join(
 		networkID      = creationParams.ID
 	)
 
+	// Start by checking that the new network isn't already joined. This
+	// shouldn't _technically_ be necessary, as the caller of Join should
+	// already be handling this, but it's worth doing to make sure we don't
+	// accidentally delete the state directory of an already joined network.
+
+	allJoinedCreationParams, err := l.Loadable(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("getting already joined networks: %w", err)
+	}
+
+	for _, joinedCreationParams := range allJoinedCreationParams {
+		if joinedCreationParams.ID == networkID {
+			return nil, fmt.Errorf("network %q already joined", networkID)
+		}
+	}
+
 	networkStateDir, networkRuntimeDir, err := networkDirs(
 		l.networksStateDir, l.networksRuntimeDir, networkID, false,
 	)
@@ -277,7 +315,7 @@ func (l *loader) Join(
 		)
 	}
 
-	return join(
+	n, err := join(
 		ctx,
 		logger,
 		l.envBinDirPath,
@@ -287,6 +325,14 @@ func (l *loader) Join(
 		networkRuntimeDir,
 		opts,
 	)
+
+	if err != nil {
+		return nil, errors.Join(
+			err, cleanupDirs(networkStateDir, networkRuntimeDir),
+		)
+	}
+
+	return n, nil
 }
 
 func (l *loader) Create(
@@ -310,7 +356,7 @@ func (l *loader) Create(
 		)
 	}
 
-	return create(
+	n, err := create(
 		ctx,
 		logger,
 		l.envBinDirPath,
@@ -322,4 +368,12 @@ func (l *loader) Create(
 		hostName,
 		opts,
 	)
+
+	if err != nil {
+		return nil, errors.Join(
+			err, cleanupDirs(networkStateDir, networkRuntimeDir),
+		)
+	}
+
+	return n, nil
 }
diff --git a/tasks/v0.0.3/network-leave.md b/tasks/v0.0.3/network-leave.md
new file mode 100644
index 0000000..0a306b2
--- /dev/null
+++ b/tasks/v0.0.3/network-leave.md
@@ -0,0 +1,8 @@
+---
+type: task
+---
+
+# Implement `network leave` sub-command
+
+When leaving a network the daemon should remove the now-defunct state-directory,
+or at least mark it as inactive in some way.